Which is the best? Well, I'm in the USAF, so I'm biased.

From the standpoint of service, you will be doing well by your country by joining any branch.
The first step I recommend that you take is going down and taking the ASVAB, to get a feel for your aptiude. I'm guessing you'd do well at about anything, but it might help you narrow down what it is you'd be good at, and would qualify for. There is a huge variety of jobs available, from being a "grunt" (Army and Marines"), to a technician who works on nuclear reactors. Then there's aircraft maintainers, air traffic controllers, Special Forces, meteorologists (my chosen field

), etc. Depends on what you want to do. If you have an idea on what it is you'd like to do, let me know and mabe I can help steer you in the right direction.
As for the different services, it's hard to describe the differences. The Army and Marines have a more hard-core structure and attitude than the Navy, and especially the USAF. Not that USAF training is a joke.....you're put under a great deal of mental stress. While our boot-camp might not be as physically intense as the other branches, your "grace under pressure" will be tested to a greater degree, IMO.
The USAF offers a generally higher standard of living than the other branches, but not always. With the Navy, you run a high risk of being at sea for 6 mos (or more) at a time. That could be cool for a person wanting to get away from their humdrum life, but not so cool if you want to settle down and start a family. In many Army and Marine units, you'll be in "the field", training a lot. The USAF does this, too, but to a lesser degree in most units--but not all.
You must also be prepared to accept the fact that you could very well be called to risk your life for your country. That's one fact that many recruiters tend to gloss over a bit. You will also lose a certain amount of personal freedom. That's not to say you're going to become a mindless drone....far from it. But there are many decisions that civilians make for themselves that are out of most servicemembers hands.
As for personal experiences, here's mine: I joined up because even though I was smart, I blew off school and knew that I could neither get into college nor have the desire to be in a classroom any more. So, I went to the USAF recruiter and signed up. I was hoping for a 9-5 job with decent benefits...no more, no less. I wish I could tell you I did it because of a higher calling to service....but I'd be lying.
So, I made it through basic OK, then my "technical school". It was rough, but I managed to squeeze in some unforgettable times. When it came time for us to get our first assignments, I was hoping for a place back close to home in Oklahoma, where I could go to work and not have to worry about anything else. So I got sent to Hurlburt Field, Florida. HQ of the USAF Special Operations Command.

THe most mobile wing int he USAF. I was scared.
Anyway, I wouldn't trade my experience there for anything. Getting away from home and going out and seeing more places before I turned 20 than most people do their entire lives was exactly what I needed. I went on to do an assignment with the Navy, then to work with the Army's 10th Mountain Division. I learned that I thrive in the situations that had scared me so bad in the past. Now that I'm back at a "regular" USAF base, I wish I could be out there with a little more "high-speed" unit.

I've come to appreciate our "core values" of integrity, service before self, and excellence in all I do. It sounds kinda corny, but I take pride in those things, whereas I would have never learned them if I'd have stayed back home.
I could go on and on. I guess military experiences will be something different for each individual, but I can guarantee that you will learn much more about yourself than you'd ever imagine possible. I don't make a lot of money and life can suck sometimes, but I wouldn't trade my experiences thus far for anything.
Sorry for rambling......I'm sure I'll be back to this thread.