I can't believe so many have listed Kimbo Slice. I think he could be interesting but he's only fought 3 tomato cans.
My favorites keep growing since I've watched EXC. I'd say Frank Shamrock, Georges St. Pierre, Matt Hughes, Chuck Liddell, Randy Couture, Josh Koscheck and Dan Henderson. I wrestled in high school so I'm partial to wrestlers. Honorable mention goes to KJ Noons. I think Noons vs. Penn would be great.