I submit:
Primer (2004)
It's a low-budget film, but then again it's perfectly fine given the story. Having seen most of the films listed here, I would say it's easily one of the best films discussing the possibility/implications of time travel.
Not to mention, the science in the film goes way over the head of most viewers (including myself and everyone I watched it with). You do get the story if you pay attention. What I like about it is that unlike most other films, including the ones here, it makes
no attempt to dumb down the material for the audience - if you don't understand it, oh well. It really gives the feel that you're watching actual scientific minds, not actors pretending to be scientists.
It's not for everyone, but it's a great film if you're up for it.