Oooh that's difficult!
I'm going to say the 28 Days Later soundtrack. Especially this track
This song is played during the climactic scene and it just fits it
so goddam well. Actually gives me goosebumps without fail every time at 2:15.
Honourable Mentions;
127 Hours - Especially 'Liberation [http://www.youtube.com/watch?v=dyrYLaVt-Lg]'
I see that LOTR has already been mentioned - I particularly love the bit from 2:00-3:15 in 'A Journey in the Dark [http://www.youtube.com/watch?v=-ttCJ6uQN_Q]'
Moon - The main track [http://www.youtube.com/watch?v=_lAfMT5FIZE&feature=relmfu] and 'Memories [http://www.youtube.com/watch?v=bIIA3NKqebg]' are great
The Inception Soundtrack. DUMMM DUUUUUUUUUUUUMM DUUUUM DUUUUUUUUUUUUUMMMM!
I'd also say Requiem for a Dream, but it's so horrendously overused now that I don't enjoy it anymore

It was on fucking
Panorama the other day!
The Bourne films [http://www.youtube.com/watch?v=N2ZExrf9ZZw]
Bladerunner [http://www.youtube.com/watch?v=JAwo7DPUFUM], obvs
Sunshine [http://www.youtube.com/watch?v=_QahzbgUVwk&feature=related]
There Will Be Blood [http://www.youtube.com/watch?v=6EhQd4nugps]
"I'm Finished!"
(Will anyone notice the joke?)
Edit:
Toaster Hunter said:
The Lord of the Rings Soundtrack. Howard shore is a genius.
Pinkie-Pie-Party said:
I know everyone uses this too often, but it really is great:
Snap!