I don't know who was "the best" I sparred with but cruiserweight Paul Jeanette is who I sparred most often. My very first sparring session was with Clarence Tillman, quite an intimidating figure to be in the ring with 6'5 or so and a solid 280 he was a strong dude.

Oliver McCall used to go to my gym quite a bit and his son may still train around my hometown....I tried my damnedest to make sure I never sparred 'The Atomic Bull'...he's older now but still in great shape, calves like tree trunks, you know that's where the power comes from.