The Star Wars Original trilogy of course had the famous revelation of Vader being Luke's father. Believed to have been killed by Vader according to Obi-Wan's tale to Luke, audiences were stunned that the central villain was one and the same as Luke's father. The revelation later on that Leia was Luke's sister had less impact, though.
The Prequel trilogy didn't really have any major twists. We already knew how things would end up, and a lot of the stuff that was "revealed" was either silly (Anakin building 3PO) or obvious from the get-go (Palpatine/Sidious).
Will there be any similar ones for the ST? Since it's heading into uncharted territory, there's a chance to be sort of fresh again. Granted, TFA already had the whole Han/Kylo thing but that wasn't really that big IMO. Many think it'll have something to do with the identity of Rey's parents, or the identity of Snoke (Heck maybe he's Rey's Dad). Any thoughts?