I used to to Taekwonoo for about four years. Dropped that and picked up Muai Thai for about two. I haven't been in a dojo since. I've always wanted to go back, but I'm easily disgusted by the whole commercialism of most dojos these days. Some of my friends go to one that practices Shotokan Karate, and they want me to join, but I keep turning them down because everyone I've met who goes there turns into a complete asshole after a month or so.
If I were to go back, I'd do something that involves weapons, like fencing. A buddy of mine has been going to a "renaissance martial arts" academy. From what I've seen so far, it's quite impressive.