For me, it's HHH and JBL.
HHH was an awesome heel in the Attitude Era, but in 2002/3 he really stepped his game up and became a great heel. He became more ruthless, sinister, cunning and colder than ever. If you thought running over Stone Cold Steve Austin at Survivor Series was worse, he put on a mask and performed necrophilia with a mannikin just to frame a PPV opponent, he manipulated a ****** and used him just to get the world title back, he screwed over many of his friends when they posed a threat to him and his world title, hell he'd probably deliver a sledgehammer to his own grandmother if it had meant getting or keeping the world title, that's just the kind of heel HHH became, he'd go to any lengths and cross any line if it benefitted him. He really pissed fans off especially when holding the title for so long and taking out the WCW legends. People wanted somebody, anybody to take the title off him. He was a unique heel, a complete badass, pure evil, he was THE MAN. And if the King Of Kings gimmick didn't speak volumes of how big his ego was then I don't know what does. Also his run and what he did with Evolution was absolutely brilliant.
JBL's transition from APA to JBL is easily one of the best reinventions in WWE history. JBL was a throwback to oldschool heels, he was so cocky & brash and was fantastic at drawing heat to the point where he also drove fans crazy and they wanted to see someone take the belt off of him so badly. He was a character people loved to hate. His promo work became better than ever aswell and he had an amazing run as WWE Champion.