I am interested in joining the Army. What MOS would most benefit me during and after service?

Depends on:
What you want to get out of your service,
what you want to do after your service, and
what that timeline looks like.

Plenty of great advice in these comments, but it is all irrelevant if you have different designs for your future ("after service").

Your fightefighter and EMT experience will help in your Army training if you go into one of those MOS, but you will start at square zero on the totem pole just like every other new recruit. After your service, your experience will help you land a job in the same category. But you already held that job... you could do one contract and get the same job after you ETS, with slightly better money, but less benefits. So, what do you want to do when you depart the service and when do you want to do it?

You mentioned in a comment to MSG (Join to see), you want to fly. Why not consider flying for the Army or Air Force? That is the only way you are getting experience in the military with "flying", unless you are dreaming of sitting in a passenger seat. Sure there are some hurdles, but the path to get into a pilot's seat is pretty well documented for anyone that wants to pursue it.

The Army prepares you for a lot of things, but what you actually get out of the military service experience is based on what you want. Being an EMT or firefighter in the military really won't get you any closer to "flying" during or after your service than you were before you joined. Being those things in the Army will make you better at them, but like I said before, you already have that job and promotions in the civilian sector in EMT and firefighter can be largely based on seniority.

If you are just focusing on yourself for discipline and limit testing, any MOS works as long as you remember why you joined and what you want to get out of it. The bad thing is all infantry experience gets you after service is... more grunt jobs (unless you supplement that with additional training, i.e. education).

Don't go firefighter in the Army. Their promotion rates are slow as hell.

Having been a Medic, it's a great MOS but promotions can be slow.

Infantry. Never been but promotion rates are pretty high.

As for what MOS would benefit you in the civilian world after the service.....any one of them could if you plan it right.

My question to you: What do YOU want to do in the Army? It doesn't matter what the Recruiter wants because he/she/they/them (whatever) are trying to make their numbers.

If you find you like doing EMt work, then medic is the obvious way to go. And if you want to really push yourself, then go for selection and become an SF medic (down the road).

But don't base your Army career on what you do NOW. Base it on what you WANT to do. The military in general, and the Army in specific is one of the very few places where you can go in with absolutely NO prior experience or expertise and be trained everything you will need to be successful (as long as you meet minimum qualifications). Literally, the sky is the limit. Yes, literally, because you can be taught to jump out of planes or fly helicopters. So... what do you WANT to do?

Great, now go do it.

Heavy Equipment operator in a combat engineer unit. Great skill and can work for any highway or DOT department in the U.S.

Cyberspace....very interesting job, very critical to the military and civilian industry

There are multiple military jobs that can transfer into the civialian sector, but there's no point in being cabable of doing a job you hate. Infantry is generic and doesn't translate very well - it does have high promotion rates if you are just looking to move up and offers you additional schools (like Airborne).
You need to consider what your interests and likes are - don't just pick a job or settle for something that is recommened to you. Your ASVAB score will also determine your options. IF something you want isn't an option the day you go to MEPS, it's okay to wait (I wish I would have - I settled and wasn't happy.)

If you are close to your bachelor's degree, I would suggest you go the officer route.

Firefighter is a great civilian job. Maybe look at National Guard so you can have the best of both worlds, guard will pay for your school as well.

If you are sold on active duty it depends what you want. If you go the CBRN route you can get on a Civil Support Team (CST) and put your fire/ems to work.

In most services, anything to do with electronic repair is a good choice.

Go to college, get your degree, THEN decide whether to serve in the military, and which branch/MOS best works for you. Going in as an officer candidate better positions you for both military and later careers.

During your service you have to do what you want to do and not what your recruiter is trying to get you to do. In 1987, when I joined, I told my recruiter that I wanted to be an MP. He said “Yeah, that’s great. Watch this movie, first” and he proceeded to show me a movie about nuclear submarines. I told him that I liked the movie, but that wasn’t what I wanted to do. After 2-3 more movies, I finally said “Ok, I’m going home now” and it wasn’t until then that they showed me the MP film.