I like the soundtracks made by the following artists and in order:
Dan Wentz
Eric Brosius
Frank Keplacki
Bobby Prince
Favorite games's sountracks though, Freespace 1 and 2 still blow me away today. I keep the entire soundtrack of FS2 on my shuffle, and FS1 on my PPC. I liked the MIDI creations as well for Descent 2. The Redbook tracks were quite good as well, prefer the first version of Descent 2 though, the one that included "haunted" by Type O-Negative, over the Definitive Collection version.
As for Brosius, System Shock 2's soundtrack, hands down the best I've heard from him. The upbeat tekky parts get you in the mood, but the low, creepy tracks really enhance the freak-out factor of that game. As for his MIDI's, I'm tied up between SS1 and Terra Nova, but I tend to like SS1's tracks, because there are a few in Terra Nova that are downright annoying.
Keplacki for C&C:Tiberium Dawn had a kick-ass soundtrack, my favorite of the C&C series. Even in Tiberian Sun, he did a good job with the atmospheric tracks.
Who can forget Bobby Prince, who made the MUS tracks for Doom? Hey, for MIDI created way back then, it was pretty darn good, and whoever did the remixes for the Doom Remake, did a decent job as well. (Dj Redlight?)
Star Wars music.. gah, I am so sick of it all, wether it be John Williams or not, I can't stand it anymore. Any new game I play that's even related to Star Wars, instantly, if the music can be muted in-game or switched off, it's OFF. I've heard John Williams's stuff so much that I cant really enjoy the music done for other films he has composed for, like Saving Private Ryan, and such. It's almost TOO much the same from piece to piece. Slowly skittering off topic, Hans Zimmer does a better job in making music for films, being able to make different types of music and still sound fresh enough to not say "oh, another Zimmer track". Just in comparison to John Williams. I guess this all means that I'm not a big Star Wars fan. Eh?
Though it's probably technically considered a port, though it's build is different than the PS2's version, Final Fantasy 7's midis, still very very well done. My favorite RPG-style soundtrack by far though over any other RPG I've played.