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
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