Punk, HHH, Lesnar, Rollins and current Daniel Bryan. I'll give a shout to Jericho as well, his BITW run was his best though.
Rollins was a good heel, feel he's better as a face though imo.
HHH in the authority, you could put Batista and Orton in there too. Batista heel run in 2010 was his best work imo.
Punk and Bryan can easily be great babyfaces and great heels. Any top babyface that go from been a crowd favorite to a hated heel is what makes them the true best. Both men have done it.
Lesnar is a natural born heel cause he's a bit dickish in real life lol.
I'd put The Miz in there, he's not a fearsome heel but he just get's heat better then anyone. Ciampa from NXT was brilliant as well.
Women's side of things. AJ Lee was a better heel then face. Alexa Bliss is another, not a great fan of her in ring work but she's a great heel character. I thought Eve Torres was such a great heel for time back in 2012. Charlotte is a great heel.
For me, Straight Edge Society Punk or if he counts Jericho's last run (which I feel was his second-best WWE run behind Honest Man). I can see an argument for HHH based on his great work with Bryan, but I would equally weigh that he played the same character for years and how horrible the results sometimes were with others.