From what I own, which is probably closer to 1/2 of the Jaguar's library, maybe less:
1. White Men Can't Jump
2. Space Ace
3. Kasumi Ninja
4. Dragon: The Bruce Lee Story
5. Checkered Flag
I tend to stay away from games that I know I won't enjoy, like RTS games or crappy 1 on one fighters, so my list may be skewed a bit since it just includes the games that I own.
I agree with the person who included I-War as a game that's underrated. I do find myself getting sucked into that one. I also like Hover Strike, although I prefer to CD version as well.
I like Highlander, but the combat drives me nuts. If I make one wrong move, there's no recovery, and the guards just destroy me.