It's between 3 guys.
Rick Rude - Awesome WWE run that didn't go as far as it should... Rude should have been winning the title but they wanted to put him with Bossman. The mark of a heel is that you know you're good and Rude walked to WCW, where he was the best heel in the business for a 2 year period bar none. He made the US title better than the WWF title, his feuds with Sting, Steamboat and even Dustin Rhodes and Chono were excellent work. The only blip was his "Halloween Phantom" debut, as they didn't think to cover his tats or give him a mask that hid his 'tasche.
That run would have continued if not for the injury... Hogan comes in? Rude would have jumped back to Vince and continued, imagine 94-95 with Rude on that WWF roster, he would have gotten the belt, feuded with Bret, Razor, Diesel maybe even Shawn and the whole landscape changes... likewise had Vince given him the belt in 1990, you would have had a much different landscape as different talent would have risen at different times from it.
Willam/Lord Steven Regal - There have been a lot of "snooty" heels over the years but none of them have come close to Regal's character. This guy screamed "better than you" with a simple turn up of his nose. Even in his earliest WCW days with "Sir William" at his side, you hated him but once they got the early WWE run out the way he became twice the best heel in the WWE... as the Commisioner being besmirched by guys like Jericho and later as RAW GM/KOTR... That was the best heel character they had had for years. Shutting off RAW early cos he could? Even Vince never pulled that kind of BS off...
Brian Pillman - I put Brian here because it was the one you never saw coming, a guy who had been a loveable scamp his entire career, even down to tiger print trunks and being a Yellow Dog but he was in reality the most natural heel of his era, when they put him with Austin no one really got it but from day one Pillman shined in the heel role, while Austin had had a couple of years honing it, Brian was new to it and out shone Steve in the obnoxious stakes. Fast forward a bit and soon the Horseman was born and he was already of that caliber as a heel he could have fitted into the group in the 80's.... and then the Cannon broke loose.
For all the Austin lovers out there, one thing is true, Pillman was doing it better and earlier than Steve... The Booker-man, Bobby Heenan's F bomb... the loose cannon was for a time the best heel character of all time, threatening to urinate in the ECW ring? Blasphemous but awesome... sadly he signed his WWE deal to be that guy and fight Bret, rolled his jeep and...no more in ring career... but he still was an awesome heel in his limited role... Pulling a gun on RAW? The Marlena "home movies"? This guy was always ahead of his time as a heel, pulling stuff that we now take for granted.
On balance I have to go with Rude as he was the only career heel. He never went face once and to me that steps him above all the others, just as you could argue Steamboat the ultimate babyface.