7. Jay Garrick Is Zoom - The Flash
The Flash has a bit of a thing for being tricksy about identity, with the identity of villains being a major selling point for the show's drama more than once. While the biggest betrayal linked to that sort of thing COULD have been the reveal that Dr Harrison Wells was actually Eobard Thawne/The Reverse Flash, its Jay Garrick's revelation that is more smartly written and thus a little more effective.
When Jay Garrick was introduced in the second season, The Flash fans were gifted a classic character from the comics who had been a forerunner as The Flash. At the same time, we were also gifted new big bad Zoom, a terrifying figure who dialled up the threat to Barry more than anyone before him. He proved his mettle by murdering Jay.
Only he didn't actually, because Zoom WAS Jay - or he was serial killer Hunter Zolomon posing as Jay - and then went on to kill Barry's father to rub salt into the already gaping emotional wounds.