It's kinda nice coming into this thread so late. I don't hardly have to think about it, I can just pick and choose from other people's lists.
However this thread says best, not favorite, and when I think of what makes the BEST heel turns, I think of ones that resulted in positive outcome for either the guy that did the heel turn or the WWE as a whole. The ones that resulted in the best outcome for the WWE as a whole take precedence over those that just resulted in a good career for the individual.
Because of that I DQ two of my favorite heel turns, with Shawn Michaels' Rockers breakup, and R-Truth's psychotic episode. Also, one of my least favorite heel turns, make it into the list of the best ever because of what it resulted in, a WM main event match for the ages.
Andre The Giant's heel turn was the biggest in WWE history, no questions asked. His heel turn set up arguably the best main event for the best wrestlemania ever. It was shocking, and Andre as a heel was straight up scary. Suddenly he went from being the gentle giant everyone loved to f**king King Kong that would go absolutely nuts and f**k up anyone and EVERYONE that got in his way.
Randy Savage's heel turn (which I did not like, as I was a huge Savage fan) pissed me off. Not because he turned on Hogan, but because his attack on Hogan made Savage split with Liz and join with Queen Sheri, and become a bad guy. I thought Hogan had it coming personally and I never did like The Macho King all that much. However, WM5's main event was epic, and was one of the best matches in one of the best feuds ever. And not only that, but without the rise of the Macho King you'd never have the reunion of Macho Man and Liz at WM 7.
Bret Hart's heel turn was another one I hated, but is definitely one of the best. As a result of HIS heel turn, Austin was made the face of the company, and set up the most controversial event in WWE history, the Screwjob.
The heel turn of Rocky Maivia, while very similar to Shawn Michaels heel turn in it's net effect of giving the guy a push that resulted in a huge career for both, was different and better than Michaels because The Rock had one of the very best careers in WWE history, and The Rock drew money like HBK only wishes he could have.
And finally the heel turn of Shawn Michaels does just barely make the list because it was just straight up good TV. It was a very scary and shocking turn, not so much that they split, but because Shawn slammed Jannetty's head through the Barber Shop window. Michaels career as a result didn't take off immediately, he spent a long time in the mid-card, and it was a good push for Michaels after having spent a number of years as a 'tag-team specialist', but he didn't skyrocket to the main event either.