A lot of classic heel/face turns have been mentioned in this thread, but too many times you could see them a mile away. Orndorf turning on Hogan prior to WM was easy to see, as was Savage turning heel in 1989. The build up was good and so was the execution, but a smart fan saw it a mile away. HBK putting Jannety through the glass was pretty cool, but those two had televised problems for weeks, as Shawn tried wrestling singles matches, and publicly blamed Jannety's interferrence for him losing a televised match to Ric Flair a few weeks earlier.
Hogan's heel turn and the creation of "Hollywood" Hogan and the NWO was totally out of nowhere. Regardless of what you can say about WCW's booking mistakes this is probably the best turn in at least the last 20 years.
Andre's heel turn before WM 3 was also phenomenal. The most beloved, and undfeated, wrestler in all WWE aligning himself with the hated Bobby Heenan family to get a shot at Hogan's title. This was a shocker, and just like the NWO turn, it produced some awesome business.
Some other honorable mentions in the "never saw it coming" category include...
Jimmy Garvin's 1987 face turn when the Midnight Express burned Ron Garvin's face - The back story portraying these two as on screen brothers had been mentioned before but Jimmy, a former co-hort of the Fabulous Freebirds, was one of the NWA's most flamboyant heel characters, a polar opposite of Ronnie's down to earth, blue jean, working man, good guy persona. Plus, Ronnie had had run ins with the likes of the Horseman and Midnights before and Jimmy never came to his aide. Of course, no one ever lit his brother's face on fire. A pay off match with the Express wasn't the money maker but the company did good business when Jimmy's pal Ric Flair sided with the Express, and eventually put his World Title on the line at the Great American Bash against Jimmy's wife and manager Prescious.
The Undertaker's turn on Jake Roberts - Taker had been a one dimensional bad guy and the prefect complement to the smooth talking Jake "The Snake" but when Roberts decided to physically assault Miss Elizabeth even The Deadman had enough. Taker's character rose to new heights after the face turn. The rest of his career has been Hall Of Fame worthy ever since.
Ric Flair breaks Dusty Rhodes leg 1985 - Flair has spent the summer doing what fan favorites like Rhodes and Magnum TA couldn't, take on "The Russian Nightmare" Nikita Kolloff, the 275 pound monster of destruction, and his uncle former WWF Champ Ivan Kolloff, and defeated them time and again in a series of bloody battles, highlighted by a 45 minute slug fest the very first Great American Bash in front of a sold out crowd at Charlotte Memorial Stadium. The Four Horsemen were not really a full fledged group and had rarely been booked together up to this point, and Flair was just about the most popular face in the company behind Rhodes. When the Kolloffs lost again to Flair inside a steel cage, they decided to lock the door and pummel the champ. The first man to the rescue was Rhodes, Flair's former rival who also hated The Russians. Rhodes saved Flair, and with fans expecting some sort of reconcilliation between these two, Flair swerved everyone and summoned the Anderson's to the ring. The ensuing beat down on Dusty drew a rabid crowd response, so brutal that security guards had to escort the Horsemen to the locker room as fans swarmed the ringside area to attack them. Arn Anderson even needed stitches after being attacked bya fan with a bottle. Dusty's return to the ring at Starrcade 85 not only produced one of the best televised matches between these two enemies, it helped the NWA reach a million dollar gate for the event, absolutley huge given that there was no Pay Per View and ticket prices were less than 1/2 what they are today.
Let's not forget the Rock becoming The Corporate Champion. Everyone thought Vince's guy in the race was Mick Foley, who was sucking up big time to the boss to be his boy. The Rock was "People's Champ" and appeared totally against alligning himself with The Corporation. The swerve at the end of that match, with Rock going over winning the World Title, was pretty surprising, a hard thing to do in this day and age.