I have several that I've played with, and each is likeable for it's own reasons:
Swords and Arrows- The RPG system that some friends of mine developed and are marketing themselves. This is what got me hard into role playing, and it was always alot of fun to see what manner of wierd shit we would come up with.
Dungeons and Dragons- The thing is huge. It has more possibilties than I ever know what to do with, but unfortunately that makes it too complicated if your DM is a rule-nazi.
The White Wolf system- I haven't played most of them directly, just Demon: The Fallen and Vampire: The Masquerade. However, with a good GM it's pretty easy to incorporate in Mage and Werewolf as well, so it's all kinds of crazy fun. Plus it's fun to play the bad-guy once in awhile and see what you come up with.
