The best face turn, is one that develops slowly over months, and occurs naturally without the booker(s) wanting it to happen. (Example: Steve Austin)
Why you ask? Because the wrestler gained the fans respect over a long period of time thus getting him (or her) a stronger FACE reaction from the crowd.
I both strongly agree, but also disagree with this. What you are describing is how a wrestler goes from being just some joe shmoe in the wrestling world to a legend, and once that ball gets rolling and starting picking up speed a faceturn cannot be stopped. Look at CM Punk lately, or Chris Jericho before that. It is the best in terms of what it means for you, the wrestler, and your career. It means you're a main eventer, it means you've really truly made it.
However I disagree too, in terms of storytelling. Mr. Perfect's face turn is one of the two best I've ever seen. Warrior's fired, Savage is looking for a new tag team partner to face Ric Flair & Scott Hall at Survivor Series. So, Mr. Perfect who has recently been cleared to wrestle again after his back injuries, is turned face by doing what you say is the worst thing you can do, turning on another heel, Bobby Heenan.
Now yeah it was much more complicated than that, but essentially that's exactly what happened. Hennig also decided to side with the biggest and most over face in the company in Randy Savage, and turning on Bobby Heenan was just about the easiest way ever to turn face. Not only that but he also made some pretty big threats towards Ric Flair & Scott Hall.
Do a youtube search for "Mr. Perfect's Face Turn" and watch all 5 parts. It was 1 episode of Prime Time Wrestling and it was one of the most exciting things I ever saw on WWE TV when I was a teen. I will never stop marking out when I watch that video and Mr. Perfect pours that pitcher of water on The Brain and calls him a wet weasel.
The other,
best face turn of all time took place in the space of 10 minutes and it was literally the best face turn of all time. I can probably say it to the well educated fans in 2 letters & 1 number and they will know exactly what I'm talking about and know I'm right: "
WM7"
Randy "Macho King" Savage is in a career match with The Ultimate Warrior, and gives it EVERYTHING he's got and still gets the s**t beat out of him, Warrior pins him with a pose and a foot on his chest to end his career. In comes his manager Queen Sherri, and starts kicking him while he's down.
Elizabeth, a plant in the crowd is watching, and can't take it anymore, she hops the barrier, runs to the ring and grabs Sherri by the hair and tosses her out of the ring. Savage is beaten senseless and doesn't even know what has happened, Elizabeth comes to his side and he swats her away.
Once the cobwebs start to clear he's looking around dazed and confused, wondering what Elizabeth is doing in the ring and why Sherri is going insane outside the ring. After a bit of doing the math (not one of Savage's strong suits) he puts it together that it was Sherri laying into him at the end because he lost everything to Warrior.
It was Elizabeth that saved him. The crowd is going nuts, Elizabeth is crying, Sherri's dragged away kicking and screaming, and Savage looks at his once girlfriend, manager, & valet, who just came from no where to support him when everyone else turned on him, and they embrace. And every woman, child, and some of the men too are crying and cheering their brains out for Randy Savage & Miss Elizabeth.
There will NEVER be a face turn that epic, that emotional, that complete ever again. That face turn alone was enough to put Randy Savage into the Hall of Fame.
A good face turn is something I haven't personally seen since Mr. Perfect, however watching old video I would say Austin coming to the aid of the WWF at the end of the alliance invasion angle was probably another one of the great face turns. And TBH I still haven't seen how Cena turned face, but, without having seen it I would bet it's probably a pretty good one as well considering he's now the face of the company.
What defines a good face turn? A good story. This is where the writers earn their money, by making a good story arch and mapping out important plot points the wrestlers need to work towards, and this is where the wrestlers test themselves as actors, fleshing out the story arch and hitting those plot points dead on.