Primer

Primer (2004) is the greatest time-travel movie ever made. It's only about an hour long, so I encourage everybody to go and watch it. I will type up my explanation of exactly what happens in the movie when I find some time. Here's a copypasta that captures my views pretty well:

Here's why Primer is the ultimate time-travel movie

• It's not the squiggle chart: As interesting and fun as it is to decipher and analyze the chart - more lines on a timeline explanation chart does not equal a better movie.
• The low budget helps: There are no big stars, no special effects, the film is set in a generic everytown. Nothing prepares the viewer for what's about to unfold.
• Shane Carruth and David Sullivan: They are everymen, perfect in their roles. They are engineers and they act like intelligent people. They don't insult anyone's intelligence. Relatable.
• Minimalist soundtrack: Relates to the budget, but the soundtrack often cues the audience what to feel. In this film, there are gaps with no soundtrack where tension rises. Brilliant!
• Innovation: Nobody has ever seen time travel done exactly this way.

Here's an anonymous comment (possibly from Shane himself) that explains why Primer succeeds despite very few people understanding the timeline chart.

The chart is endearing but ultimately irrelevant. Primer is about making you 
think you know the rules, then changing the rules. We didn't want you to make 
that chart, we wanted to convey that feeling that something like that chart 
existed, and you couldn't figure it out. For every nerd who figured out that 
chart, there were thousands of normal people who felt that thrill that the rules 
had been redefined. That is the ultimate victory of Primer.

The director's commentary is a little difficult to find online so I can send it to you if you shoot me an email. Well worth the listen.