What is the best route to become a military pilot? Warrant or 2LT? Which branch?

During army flight school they say "choose the mission, not the airframe". I think that needs to be the same for choosing which service she wants. Probably the first decisions she needs to make are:
1) Does she want to be Active Duty or Guard/Reserve
2) What type of missions does she want
3) What type of airframe does she want (type meaning fixed-wing, rotary-wing, or tilt-rotor)

If she wants to fly helicopters, then it's a pretty easy decision on the service (from my perspective), Army, unless she's looking to do sea duty. Army has by far the most extensive helicopter fleet. If she's looking to do tilt-rotor, i.e. V-22 Osprey, then again, it's a relative easy choice because only the Air Force and Marines have tilt-rotor craft. (Beware though, tilt-rotor time only counts for tilt-rotor time, i.e. not FW or RW, so it's fairly useless in the civilian sector... at least for now) If she wants fighters, then AF, Navy or Marines

Army: Predominantly RW and predominantly a local area missions. Army FW is predominantly MI on the AD side and predominantly OSA on the NG/Reserve side (more Reserve than NG... and you'd be surprised how much flight time they actually get. Most NG/Reserve average at least as much as AD, at least in the Army). Army is also now "FW for life" so if she assess as a FW pilot there is no chance to go back to RW (like there used to be) and vice versa, you can no longer apply for FW later down the road if you are a RW guy. Army also has the 160th SOAR (which I imagine is open to females now since they opened everything else up).

Navy: Sea duty. Unique aspect that only they can really offer. I have done some deck landings on Navy ships before, but it's definitely a cool little extra as an Army pilot and not a "norm" for our mission. Also, more of a regional mission. They have larger planes than the Army but they're still staying more or less in one region/geographic command.

AF: Potential worldwide missions, depending on the airframe. The only service with true "heavy" aircraft. Also unique aircraft like the U-2, B-2, and B-52.

Marines: Uncertain as to their actual mission so I defer to any Marine pilots out there. I think the largest aircraft they have is the C-130. Mostly fighters and helos as far as I know.

Coast Guard: Pretty much all rescue operations. Certainly the smallest fleet of all the services. Plus side is you're staying stateside. Down side is you don't really have the opportunity to deploy or be stationed overseas. I've heard though that because they're so small pilot's can be retrained on different airframes when they PCS. (Are there any CG pilots out there that can confirm this?) If that's true then that's certainly an advantage as opposed to the other services that you are trained on a specific airframe and pretty much stay that for your career, save for any special assignments that require special training.

As far as Warrant vs. RLO, only the Army has flying Warrants anymore. As others have said, at least on the Army side, if her goal is to stay in the cockpit then Warrant is the best choice. RLOs get little flight time after CPT, and those really only get it if they are lucky enough to be selected for BN/BDE command of an aviation unit. If they are selected for command of a unit that does not have organic aircraft or are doing staff work outside the unit then they will likely not be able to fly. Warrant also has the benefit of applying for the WOFT program as a civilian, so she would know if she was accepted to flight school before ever joining the Army. In the Guard and Reserve she would also already know what aircraft she was going to fly before going to flight school (for WO and RLO alike).

If she's looking to fly in the civilian world after her initial commitment then I think any service would be good because the majority of your flight time in all services is toward the beginning of your career before you "promote yourself out of the cockpit". Employers prefer recency (within the past 12 months) in the cockpit when they hire.

Well, that was much longer than I intended, but hopefully it gives some good thought points. I think each service is strong in certain areas. At the end of the day, they type of flying you want to do is more important than what airframe you are flying because that is what will make you either love or hate your work. Any other questions, feel free to ask.

Speaking from someone who has been in the Air Force and the Army, here is my advice. First, she should decide what her ultimate goal is? Obviously she wants to fly, but does she want to be a career military person and retire? Does she just want to do the minimal amount of time in the military and then fly in the civilian world? Does she eventually want to get married and have a family. In the Army, being a Warrant Officer is probably the quickest way to guarantee a full career of flying. However, the Army is very deployment heavy and doesn't leave much room for families. The Air Force doesn't always guarantee a pilot job. Some have hopes of being a pilot, but end up being a flight engineer or other flight support job, but not actually a rated aviator. Air Force treats their people better, they have nicer bases, and deploy for shorter durations than the Army. Looking back on everything that I've done, it would seem like flying for the Coast Guard would be the ultimate. Much fewer deployments, if any at all. I can't speak for any Navy or Marine flying. Being a female, I can see Marines and Navy not being quite as female friendly as the Air Force or Army. Not to say anything bad about the Navy or Marines, but if I was locked up on a boat with a bunch of dudes all the time, I'd probably be dry humping anything that resembled a female. I think the Army and Air Force do a good job with all the SHARP stuff. I saw many females successfully navigate Army Warrant Officer Candidate School and Flight School. That's my $.02 and feel free to ask me any questions.

If she wants to be rotary, the Army is a good choice. You can apply (it's a long process) with no prior service, no prior experience, and if selected, attend BCT, WOCS, and flight school which is broken into about five sub courses. With that she's at least guaranteed the branch if selected. She could, depending on how she does in flight school, select fixed wing. There have been slots but usually go to top of the class. Here is some information on the application process. Best of luck.

http://www.usarec.army.mil/hq/warrant/prerequ/woft.shtml

Here's where I'm coming from: I was an Air Force pilot for 22 years and flew for 16 of those years. I taught pilot training for 3 years. I also worked as an Army Civilian for 6 years.
First, your daughter should complete her degree. No matter what her choice is it will stand her in good stead to have a bachelors degree.
There's only one Service dedicated to airpower including all types of flying, the Air Force. If she's interested in a military career, she'll fare better as an AF pilot than anywhere else. The pathway to senior leadership is fairly straightforward for Air Force pilots. You'll notice every Air Force Chief of Staff has been a pilot. The pyramid of opportunity narrows significantly as rank increases beyond O-4, because there are only a few flying units to command, and that opportunity is limited to rated officers only. (Non-rated and rated officers can command non-flying units, but the path to being a general officer frequently goes through commanding a flying unit.) Being an Army WO is a respectable career path and I knew WOs who loved their jobs and were excellent pilots. This path does not lead to command. That's the way the WO track is designed. WOs get to fly almost exclusively throughout their career and that can be a plus. Commissioned Army Aviators may advance to command, but many I saw transfered out of Aviation to advance their career. Although Aviation is considered a combat branch, I don't recall seeing a Chief of Staff of the Army who was strictly from Aviation. The Navy is similar. Naval aviators (including Marines) are on a track to advance within aviation, but will have to branch out if they want the top positions. Aircraft Carrier Captains are aviators, but they generally have surface warfare experience also. Again the opportunity pyramids narrow significantly above O-4 because of the small number of flying units.
If she's wanting to get the training and serve the minimum commitment so she can fly in the private sector, then I believe the Air Force is the best way to go also. Only the Air Force has the large multi-engine jets that are similar to what the airlines fly. She can volunteer for tanker-transport path pilot training, serve her commitment (I think it's 5 years), and have experience and flying hours that are attractive to the commercial aviation industry.
The competition for Air Force pilot training slots is keen. She nearly has a bachelors degree, so she should talk with an AF Recruiter about opportunities to go to OCS and pilot training. Commissioning source made little difference in the success rate of the pilot training students I taught. After the first month or so, it was difficult to tell which LT had gone through OCS and which had graduated from the Air Force Academy. Pilot training is difficult, but most people who are physically qualified and well motivated get through. There are a few student pilots who just can't get the hand-eye-brain coordination together to fly safely, they are eliminated. A few can't handle the academics, but it's really no more difficult than the undergraduate work they completed to get their commission. Academic issues are usually self-discipline problems (you do have to study). Adapting to military life or family problems put additional pressure on student pilots, but few washout because of these stressors. (Caution: If you complete AF OCS and subsequently washout of pilot training, you probably still have to serve at least a minimum commitment, probably 4 years, as a AF officer.)
Overall the Air Force offers the best opportunity for flying and potential advancement in a military career. It also provides flying experience that is attractive to the airline industry. Go see an Air Force recruiter.

SSG Stokes-The one consistent thing about military aviation pipelines is that they have high attrition rates. The Naval Aviation (to include Marine Aviation) syllabus is probably among the most difficult, as Naval or Marine Corps fixed-wing Aviators have to do everything an Air Force pilot has to do...plus do it from a carrier at sea. I've no experience with the Army Aviation program, but I've enough friends who went through it to surmise that the aviation specific challenges are no less arduous.

Personally, I feel the most "dicey" way of going about it is to pursue a full commission through ROTC or the Academy before wining a contract. You've four years of intensive study, physical training and leadership development before "jumping off", and then, as much as another two years before you get to a squadron...that's a long road with lots of chances to end up losing your career along the way.

OCS is probably the best way heading into the Navy...if flying is what you want more than anything else. She'll get to the VTs faster, with more energy available, so to speak. However, there are downsides in terms of comparing that route in terms of the overall career.

Ultimately, flight school is tough...real tough. If someone is uncomfortable in the water, struggles with mathematics or sciences, and doesn't have excellent memorization skills...it's very long odds making it through all three phases of SNA or SNFO. I had plenty of Air Force students doing the water survival portion alongside me...so I don't know that there's any factor of the first two phases that would differ much between services. Most of the guys I know who made it past Intermediate didn't choke out on the boat...we lost most folks at API or Primary.

One thing I'd have done different? Getting a private pilot's license on the civil side would've informed me a great deal more about the challenges of flying, navigating and communicating...and helped me know which weaknesses to focus on most. I entered the program expecting API's physical demands, particularly water survival (I wasn't a strong swimmer) to be my nemesis...I handled those pretty well. Doing fuel consumption calcs while talking to two controllers and short-tempered instructor pilot proved more devastating for me personally.

Like others have said, it depends on what she wants to fly. For fighter pilot wanna-be's, there's also a physiological component...if she's a runner type, tall, thin and low blood pressure, high G's may not be her thing. I'd recommend a flight in a high performance acro plane at your local airport to see if she likes acrobatics and pulling G's. About 90% of us went in to UPT thinking we wanted to fly fighters, and after a year of yanking and banking, only about 30% still thought it would be great for another 8-15 years, so it's not for everyone. Statistically best odds to become a (fixed wing) pilot is for USAFA grads...higher entry rate into UPT than for any other service or commissioning source.

SSG (Join to see) I'm biased, but if she wants to be the best of the best, suggest Carrier aviation. She'd be an Ensign, though. :)

It all depends on what you want to fly. First off, you an not just come into the Army as an Warrant officer. If she wanted to join to go into aviation she would have to go the commissioned route. It is difficult to get aviation. Nothing is saying she can't but it is very competitive. If she was wiling to join the Army she would be in helicopters. The only way to go warrant is to go enlisted and put some time in and then apply. A lot of soldier prefer to go aviation as a warrant due their ability to fly much longer than their commissioned brothers.

If she was going to the AF she would have to go commissioned. As stated they don't have warrant pilots. Keep in mind that they Navy and Marines both have aviators. The Navy has a pretty good share. I am not sure about their aviation programs but I wouldn't count them out.

I am SERIOUS: Go United States Coast Guard. Male/Female integration is 100% across the board. Same money, same ranks as Navy, same military retirement, same in-service benefits as the other four services, and like a very large family. Best deal out there, bar none.

If she wants to Fly and just only fly especially rotor the she needs to go Army Warrant. If she wants to fly fixed wing (fighters), then she needs go AF, Marines, Navy. If she wants to fly fixed wing heavy aircraft and build up hours for commercial ratings (airline)...definitely Air Force...lots of hours...especially for young aviators. For the first few years of flying on any of these she will build up hours, especially if she is single and can go on every thing smoking without ties. Good luck.