I always find it tricky to just pick a singular favourite, so instead I'll list a few here :p
District 9 has always been a film that wows me, I just wish I could learn even more about the prawns :]
Mars Attacks never gets old, if you're someone who's into behind the scenes trivia there's definitely a lot to read into a little with this one!
I don't often watch "newer" films but Colour out of Space absolutely blew my mind, I'm always big on the way colour can be strategically used in film and this was a huge treat to the eyes.
[Edit: Accidentally hit post before I was done]