#7 Bret Hart and Shawn Michaels
Here's a feud that boiled over into the infamous Montreal Screw Job.
Bret Hart was the main star of WWE after Hulk Hogan's departure. HBK was an up and coming talent who had recently switched to the singles division after years of teaming with Marty Jannety in the Rockers.
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The two immediately developed a dislike for each other. Bret found Michaels to be unprofessional and arrogant, while HBK thought Bret took himself too seriously and acted like he knew everything about wrestling.
Nonetheless, the two put on some of the best matches of their careers against each other, creating magic in spite of, or perhaps because of, their real life friction.
After the Montreal Screw Job, Bret swore he would never forgive HBK, but years later on Raw he did just that.