Bobby Heenan for managing what is probably the largest group of wrestlers for one time. His stable went all the way back to the days of the AWA. Heenan is largely tied to a very important part of wrestling (the Golden Age/Rock N' Wrestling Era of the 80's), as Hulk Hogan spent most of his prime feuding with mostly just different members of the Bobby Heenan Family (King Kong Bundy, Andre The Giant, Rick Rude, Mr. Perfect, Greg Valentine, Big John Studd, Harley Race, etc.)
Paul Bearer is a close runner up. Like WWE said the week after his death, he's probably the most unique manager because he managed the most unique individual in WWE history. Most all managers do the typical promo work along with the talent they manage, but Bearer looked creepy and carried an urn that Taker supposedly got his "powers from" and took part in vignettes and videos of Taker preparing coffins for his opponents and stuff. Later, he would branch out and manage other bizarre guys like Mankind and Kane, one who pulled out his own hair and hurt himself physically and the other who was a physically/mentally scarred half-brother of the Undertaker himself. He's taken a whole trio of freakish individuals under his wing.