Ah I do love to make a rant out of an easy question.
CM Punk built the name that he did in the indies through this "Straight Edge and Better Than You" gimmick. He was a heel for the majority of his time in ROH and various other promotions, and he became one of the best promo men in the industry (independent or otherwise). See his story of the Old Man and the Snake for proof. He was a masterful heel and it got him signed.
He debuted in the WWE as a face and rose quite quickly, capturing title after title. He may have been one of the best starters in recent memory, winning the ECW, Intercontinental, Tag Team and World Title, as well as two Money in the Bank wins in a short space of time. He was over, people liked him, he was marketable.
But there seemed to be a level of resistance to him that started during his poor world title run. People labeled him a paper champion, a failure, a disappointment. I've witnessed crowds turn on him live, booing him, even saying he can't wrestle. His positive reactions dipped week after week. The WWE seemingly abandoned him mid-reign and gave focus to others. He was never pinned to lose his title and then embarked on a fairly dull journey to a second MITB win.
It was when he beat Jeff Hardy, creating another legit Money in the Bank shocker, that Punk really hit his stride. His transition from clean-cut (minus the tattoos) face, to conniving heel was wonderfully executed. At first he was rational and honorable as he feuded with Edge & Jeff at the same time, but gradually it became clear he was making excuses and his true nature was revealed to us.
Jeff Hardy vs CM Punk was the WWE feud of the year in 2009 in my opinion, with incredible mic work from Punk, and some very good wrestling from both men. The angle wrote itself, and Punk was drawing major, MAJOR heel heat. Retiring Jeff and then dressing as him the following week drew a line under it in my mind. Punk is better as a heel than a face.
Oh but wait, there's more...
He started the Straight-Edge Society, becoming a cult leader, a savior, a messiah. He riles the audience up and hypnotizes the I.W.C. Him cutting promos in the Rumble and Elimination Chamber was a stroke of genius. Every time he chastises the audience for abusing alcohol and pills, they boo ever-louder. He has a posse to back him up, and he's moved from Jeff Hardy to Rey Mysterio, targeting the child-friendly characters, whilst preaching to the adults.
He knows precisely what to say, and you cannot deny that when he is on the screen everybody's eyes are fixed on him. Nobody is playing their character better in wrestling anywhere in the world right now. Nobody. Jericho gets face pops for being too good, but Punk is getting booed week after week, whilst still putting on great matches.
No contest in my mind. Punk is at present the best heel in a company full of great heels.