Which is the best branch for reserve/guard duty?

Army guard has the most opportunities for advancement/experience. If your friend ever does hit a snag with rank advancement, all would be necessary, is a move to a new unit. Or so has been my experience. Plus there are many more "Volunteer" opportunities. I would suggest your friend sign up for a combat group though. 11B is a good place to start.

I can only speak for my Army experience.
I chose the reserves over the guard because I never wanted to put myself into a position where I might have to suppress my neighbors during disturbances. I joined the reserves as a prior-service electronic technician at the age of 33.

I note that your friend is age 37. (S)he may not be eligible for retirement, if (s)he has insufficient prior-service. Check with the recruiter.

Air Force Reserves

Air Force Reserve. I think the cutoff age is 35 or 36 tho. A waiver can be obtained

That's a loaded question. As a two service member (Active duty Army and Air Force Guard & Reserve), I can tell you that each service is vastly different. It really depends on what he wants to do and his personality. At 37 years old, your friend is probably set in his ways much more than an 18 year old. He may have a hard time adjusting to the Marines who tend to be pretty rigid in their ways. The Guard (both Army and Air Force) tend to be more relaxed than their Reserve cousins.

Depending on the MOS, the services have different opportunities. Both the Army and Air Force have Reserve/Guard Special Operations units if he wants to be real gung ho.

I suggest that he do his homework, understand that recruiters lie, and talk to as many Reserves from the different branches as he can before he decides.

http://www.militaryspot.com/enlist/what-is-the-maximum-age-limit-for-each-branch-of-the-military

I found that site hope it's useful. As mentioned here, obviously, knowing more about background, interests, ambitions, and education, would certainly help to answer, of course. Themusphs commissioned corps, for those with clinical or, e.g., engineering, I'dread can, I think, go to 44, however, as I'd said, you might need to describe more, to get more specific answers, that's all I'm saying, hope was of help, would be eager to hear more, many thanks.

At 37 I don't think he can join any branch unless, he has some degree in medical to become commissioned.

According to the national guard and reserve almanac the highest end strength of service members are in the Army National Guard which means more slots or promotions more opportunities for you

Yeah your options are kind of narrow. I know the Marine Corps Reserve stops taking people after their 29th birthday unless you have a special waiver. The Air Force doesnt stoo until your 39th birthday; So, air force would probably be the only option. I'm not really familiar with the rest.

Guard bro trust me I was in the guard first now I am in the reserve don't do reserves I am trying to get kicked out of the reserves right now then I will go back into the guard