Muay Thai.
Agreed. Or if you want a probably more strenuous workout but with, in my only slightly educated opinion, more chance of getting hurt, Brazillian Jujitsu. Also awesome: Aikido if you can find a good teacher. Again, though, you're probably going to get hurt every now and then.
Tai Chi is for when you're old IMO. Not that it's not great, but why not wait until you're old? It's probably the best one to do when your body starts to get a bit worn down.
On a side note:
Wow is this guy awesome. It's very questionable how he would stand up to some of the premiere fighters in the UFC, but I love his style and that he actually makes a style of kung fu work in the UFC. He's just a great and talented fighter though, I think that's why he can pull it off.
His discipline (Sanshou) also would be a great choice if you're into that kind of thing.
That all being said, I think Karate and Jeet Kune Do are also great choices. I just personally would rather do Muay Thai right now if I had the time.
edit: I forgot. If you're watching that match, it should be noted that Frank Shamrock's forearm fractured in the first round and broke in the second. His right arm I think? And he still gives Le a run for his money in the 3rd. So, bottom line, Le probably can't really compete with the best mixed martial artists, even though I love watching him.
But seriously, he lands stuff that nobody lands, and with pretty good consistency. He throws flashy stuff, but he's careful about when he throws it. He also generally protects himself well.