Corey Sanders...sparring partner for the likes of the Klitschkos, Mike Tyson, Evander Holyfield, Oleg Maskaev, Tony Tubbs, Frans Botha, pretty much every top heavyweight in the late 80's / early 90's...!!!
Emanuel Steward says that Bernard "Superbad" Mayes used to kick Tommy Hearns butt every day in sparring. The guy drank too much and died of cirrhosis in his 30s.
There are guys out there who's greatness never translated on fight night but were/are a handful for any fighter in the gym. Rpy Williams is a perfect example of that. I first read about him in Larry Holmes autobiography. If you dont know him, look him up. http://www.independent.co.uk/sport/general/boxing/steve-bunce-roy-tiger-williams-once-muhammad-alis-sparring-partner-wont-be-celebrating-rumble-anniversary-9824283.html
Roy Williams was a big ex-con. He was probably a little overrated though, his main feature was that he was 6-5 240lbs. in the 1970s.
James Toney has sparred both Klitschko brothers as a LHW and he did good. He didn't dominate them but he did good. Keep in mind Toney is only 5'10" and he was a LHW back then. That was back in 1996 when he was preparing for the Montell Griffin rematch. Both Klitschkos were at the beginning of their respective pro careers, and Wlad had won the olympic gold medal that year.