In no specific order...
Eddie Guerrero: That Rey Mysterio and "I'm your papi" rivalry was classic. No one else in WWE can pull of that type of seriousness. Those moves he did in the past ladder match with Rey were brutal, like three amigos on the ladder.
CM Punk: This guy is a true rebel and pull off a heel turn by the snap of his fingers. The Cena vs Punk rivalry is a timeless classic.
Edge: He legitimately was the Rated R Superstar with the Cena rivalries and Foley Rivalry. Spear through a flaming table equals one of a kind moment
Randy Orton: He was great heel as the Legend Killer. Trying stomp on the legacy of every legend he faced. He took it to the extreme as the Viper. Changed his in ring style becoming more stealth and predator like.
Chris Jericho: Jericho make it easy as a headlock to be a heel with his promos and in-ring style. He is so charismatic with it you cannot help but to be in awe.
JBL: every time he defended the WWE Title I wanted him to loose. With him being one of those clique or faction heel leaders I couldn't stand him as a kid
Triple H: Don't get me started on how many people he buried. With Evolution they were the New Age Four Horsemen to a lot of fans. When I was a Kid Triple H was like the Devil and Flair, Batista, and Orton was like his demons. Now when I look back They were one of the most legendary factions of all time. Also Triple H is one of The Most Legendary Heels of all time
Hollywood Rock(2003): The Rock 2003 heel run was awesome. The Reason why because a lot of people say The Rock turned on us by going to Hollywood, but some say the people turned on The Rock by not supporting him. Summer Slam 2002 is evidence enough. So what The Rock and WWE did was make a Rock gimmick where he fires back at the crowd for what they did. Those moments and segments were classic.
Brock Lesnar: Brock Lesnar is a good heel or tweener now, but back in his early days he was a hardcore heel, especially with that Real Brock Lesnar stint. Broke Hardcore Holly's Neck, do bearhug that could squeeze blood out of you, Destroy jobbers like no tommorow, Broke Zach Gowen's Leg, and more. Brock back in the day was way more heel than he is now.
Miz: Truly I can barely stand this guy. So annoying. Everyone was shocked when he was main eventing WM27 but there is a first time for everything.
The WWE Fans: These guys have ruined so many matches and events it's not funny. The fans now have to be the worst fans ever. They got too far by making a hash tag called"[HASHTAG]#KillRomansDaughter[/HASHTAG]". They go as far as one night on RAW chant "You Suck Bryan" at Brie, "You Suck Tyson" at Natalya, "You Suck Rusev" and more. The fans nowadays has got out of hand. Their only few true wrestling fans left and it looks like we are it
That is my list