I might as well do this since I’ve been on this topic for a bit already. Note that this list is in no particular order, even though I may favor one over the other. The order is only determined by the order that I thought of them in.
1) Hans Zimmer
Zimmer is a well-noted composer all around; he’s uber prolific, having done an absolute ton of movie soundtracks and video game soundtracks. Among his noted soundtracks include: the most recent Batman trilogy, Crysis 2, and Call of Duty Modern Warfare 2.
Here’s a song from Modern Warfare 2 that can give you an idea of what Zimmer is like in the video game medium, if you’ve never heard a video game soundtrack composed by him: http://www.youtube.com/watch?v=8D527OOb7MU&playnext=1&list=PL4C120883488DDE14&feature=results_video
2) Jesper Kyd
Jesper Kyd has composed mostly video game soundtracks throughout his career. As far as I know, he has only done soundtracks for small cinematic projects. He also has an odd habit of doing soundtracks for games that aren’t necessarily that popular, which I think is a shame. He is truly versatile and his strongest suit is perhaps his ability to create ambient background music.
3) Gerard Marino
Marino is most famous for his works toward the God of War series of games, especially the first two titles that were released. Quite frankly, I love this guy for giving the games such a mythological atmosphere through his music. He even had a choir singing in Greek for his works, which I think is impressive for a guy who doesn’t have extensive knowledge in the subject and had to get advice from a translator.
4) Cris Velasco
Velasco is another prolific video game composer who has also contributed to the God of War series of video games. I know his work particularly from the first Darksiders game and the first Prototype game. His work is all-around epicness.
A song from Cris Velasco that’s featured in the Prototype original soundtrack: http://www.youtube.com/watch?v=U6Z7BYK_E5o
5) Gustaf Grefberg
I know Grefberg mostly from his soundtrack work on The Darkness, in which he uses choir and rock elements to produce a really epic hybrid mix. He’s also done work The Chronicles of Riddick: Escape from Butcher Bay.
This song is probably my favorite overall from Grefberg: http://www.youtube.com/watch?v=RKzbAG1Jyo8