I've already chosen kung fu, because I like the logical basis of it. The movements, the goals, the thinking, everything makes instant sense to me, without needing explanation. It just seems instantly practical for fighting, rather than all the mystical BS that some schools like to give you. That may just be a function of the teacher, though.
And to be fair, I have very little experience with the others. I've tried aikido and left after 2 months because I hated it so much (incredibly slow pace, unintuitive movements, didn't seem like it had any practical use) despite all the good things I'd heard about it.
edit: next on my list to try is jui jitsu, and I'd love to check out muay thai eventually as well. Most of the martial arts intrigue me, actually.
edit2: capoeira is really cool. I saw a demonstration once, it looked like a lot of fun (and hard work!).
I'd actually eventually like to, though so far I've been too lazy. But Brazilian Jiu-Jitsu is what I'm aiming for. Why? It seems to be a more advanced and functional martial art, what with it springing up only in the past century.
Brazilian Jiu-Jitsu. I wrestled in high school and a little in college and jiu-jitsu seems to incorporate everything into it. I really enjoyed it and would love to find a gym (and time) to be able to learn.
I don't currently practice any martial arts and I think I'd need to get in a lot better shape to do that than I am now, but I chose Aikido, buceause I like the idea of using someone eleses offense against them (and I have considered taking courses in it from time to time).
Ninjutsu - mostly because I did study it for a year or so. I found it very practical, very non-flashy, very noncompetitive, and very analytical. It has a rich history, and although it is spiritual to an extent, there's really not any mumbo-jumbo type stuff. It's primarily about balance, movement, subtle manipulations and straightforwardness in thought - and that applied for really everything from non-weapon fighting, to weapon training, to acrobatics. The only reason I stopped is that my knees just couldn't take the extended periods of putting all of my weight on bent legs.
One thing it's not is a fast-moving (until you get good, which takes a LONG time - I never made it that far) sport-type martial art. The basic attitude is along the lines of "fighting's not a sport - don't fight if you can avoid it, but be prepared, and if you can't avoid it, fight to win, at all costs."
All in all, I enjoyed it (even if I wasn't all that great at it). It definitely helped my flexibility, and even a relatively clutzy guy like me noticed that I could get at least a semblance of grace and balance if I worked at it. The environment was good, the people were by-and-large easy to get along with (although *not* chatty - in type-terms I'd have to say it was a very ISTx crowd and philosophy). Sort of miss it .