Interstellar
The third act of Interstellar is a mind-boggling one. First, Mann (Matt Damon) attempts to murder Cooper (Matthew McConaughey), cracking his helmet and leaving him to die. However, things go awry, and Cooper survives—before getting sucked into a black hole and entering a liminal space between all five of the dimensions. He manages to pass on critical information to his daughter, Murph (Jessica Chastain), who uses it to solve an equation that will allow humanity to safely leave Earth before it dies.
Saw
Many fans believe that the first movie in the Saw franchise is also the best. The film ended with two reveals—Zep, the implied mastermind behind it all, was blackmailed into working for Jigsaw. Secondly, the body in the middle of the bathroom was very much alive, eavesdropping on Dr. Gordon and Adam as they uncovered darkest secrets. Oh, and the "body" was none other than Jigsaw himself.