I'd like to amend this list to include just one name:
This is definitely a broad question. Here's my short list:
Fedor (best Sambo)
Hidehiko Yoshida (best Judo)
Big Nog (best BJJ, at least from an MMA standpoint)
Josh Barnett (best wrestling)
BJ Penn (as far as lightweight grappling)
Xande Ribiero (strictly sub-grappling competition, but he's the best in my opinion)
Rulon Gardner (Olympic Gold Medalist: Roman Greco)
Roger Gracie (gotta be one on the list)
That's just my input.
His performance at Mundials this year was the greatest single performance by a grappler in the history of the sport. I have no doubt that the guy's the best that there is.
That said, I love watching his matchups with Xande Ribeiro (who I consider a close second).