My favourite sci-fi film (and favourite film in general) is The Fountain. It's a really special piece of work. It didn't have a big budget, but the special effects are beautiful. Clint Mansell's score is stellar, and Darren Aronofsky's writing in this film is my favourite out of the ones I've seen by him. The actors' (especially Jackman's) performance were absolutely fantastic, too. It's a quite special story and it's not for everyone, but I would hope everyone gave it a shot. As for post-apocalyptic, I think my favourite would be The Road. The book and the film are both great and sort of very emotionally engaging, very dark and very depressing.
The Fountain is amazing, it gets quite a bit of hate from what I've seen though and has become somewhat of a "cool to hate" movie. Clint Mansell is just fantastic at scoring, as evidenced by his score for Moon; he really made those movies. If you haven't seen Aronofsky's Pi you really should.