Kevin Owens Betrays Chris Jericho
This wasn't actually a heel turn, because Owens and Jericho had spent many a month as a heel duo, keeping a tight grasp on the Universal Championship. They teased a Jericho babyface turn multiple times and had the crowd in the palm of their hands as it always turned out to be a ruse. They fooled the fans, and they fooled their opponents, who expected Jericho to finally say "enough is enough" when it came to being treated like a tool for Owens to continue to claim victories.
This all led to Chris Jericho's infamous Festival of Friendship. It had become more and more obvious in the weeks leading up to this event that Owens no longer had any use for Jericho, but Jericho could not read the writing on the wall. Jericho presented Owens with gifts, a magical painting, and a wonderful party. Owens gave Jericho a gift as well -- the List of Owens, and the only name on that list was Chris Jericho. It was the outcome of that segment that everyone expected, but it was as perfect as a betrayal could possibly be.
Stephanie McMahon Turns On Vince McMahon and Joins Triple H
This heel turn was anticipated by many fans as well as pundits and writers on the internet at the time, but it was mostly speculation. Based on WWE's predilection to do "swerves" in which a character who seemed to lean heavily to one side suddenly jumped to the other, Stephanie betraying the trust of her father became a likely, but not guaranteed, occurrence. At Armageddon 1999, Vince McMahon battled Triple H in a No Holds Barred Match.
A Triple H victory would give him a shot at Big Show's WWF Championship and force Stephanie to remain married to him. If Vince won, the forced marriage between Stephanie and Hunter would be annulled. As it turned out, Stephanie McMahon was in on it all along, because she wanted to overtake her overbearing father and gain control of the company, and knew that siding with Triple H, the conniving heel, would help her do just that. The turn changed the WWF forever and while many people expected that it could happen, they were still unsure until it actually did happen, and it remains an amazing moment to this day.