It depends on what grappling rules you are referring to. BJJ rules, I'd say Jacare, Roger Gracie, Xande Ribiero, and Marcelo Garcia are the best right now.
Sub-Grappling would be Jacare, Roger, Marcelo Garcia, Xande, Lister, and Arona.
MMA grappling would be Mach Sakurai, BJ Penn, Arona, Lister, both Nogieras, and Busta. Actually there are alot of good judo grapplers entering MMA lately too. Nakumura, Yoshida, etc.
I don't keep up with the judo world, but you could look up the latest Judo world champions, and Olympians.
Also, don't forget Freestyle, and Greco Roman Wresling.