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.
Sig by Toxic
Barnett - Toquinho -Werdum - "Nurmie"
Z. Gurgel - Morango - Rocha - Tiequan