Annihilation
Annihilation is a wild movie that will definitely keep you guessing throughout the story. It follows Lena (Natalie Portman) as she enters The Shimmer with a group of women who are there to help her defeat the evil that lurks within. In the end, she's fighting with her doppelganger. She apparently makes it out alive, but her eyes still shimmer, which indicates that it's not actually her. The director has said that it is her, but the shimmer is within everyone already.
Inception
This classic movie ending as everyone talking about it even 10 years later! In order to find out if they are dreaming, the characters use an object. For DiCaprio, he uses a topper. If it falls, he's not dreaming. The end scene shows the topper spinning and spinning, but it never falls and the screen cuts out as it wobbles. The director has stated that they are indeed in the real world and that he isn't dreaming. There are other clues to confirm this.