I'm partly with Lewieroo0... the C&C series music was epic, and (for purposes of grammys etc) could easily have come off a major movie soundtrack ... if not for that whole issue of being 22khz mono ADPCM 'n' all (Q: are full CD quality soundtracks available of that, same as with Halo?). I extracted them and burned to a couple CDRs at one point and it's good stuff even if it sounds like it's coming out of an AM radio.
Other than that I haven't paid a massive amount of attention to the non-demo game-music scene for some reason so my standouts may be a bit wierd
8-bit: Sonic, Streets of Rage (i'm sorry - the NES soundchip just grates on me unless it's used EXCEPTIONALLY well, so it's got to be Sega), honourable mention for Link's Awakening. Using the tech to its proper potential but also good pieces of music that fit the mood well.
16-bit: Xenon (the first one), Final Fantasy 6, Lemmings (well, of course) and the title to 7 Gates of Jambayla (it's a total mismatch, but I knew it as a standalone piece before finding out about the game and liked it a lot, and can be sometimes found cranking it on headphones with lots of bass boost for 2 or 3 repeats if I've got something heavy to work on). And probably a whole load of others given I cut my teeth on the 16s, but I just had lunch and my mind is dim.
((honourable mention: honkin' great piles of game-inspired YM2192 chiptunes))
32-bit & up: Wot, no GTA? Particularly the fake radio stations and purpose made music. They're a coffee and cream combination ... good by themselves, great together.
Other contenders using licensed music - Gran Turismo 3 + 4 (pretty much just the bespoke mixes of Just A Day and Reason Is Treason that are hard as hell to get hold of without recording off the PS2's lineout, but Moon Over The Castle is a catchy piece by itself) and SSX. Like another poster said, it got me into a few bands (which others e.g. Tony Hawks didn't manage), plus *most* (not all) of them fit their particular track and the game in general very well... and regardless of anything else, kicking Finished Symphony fully into life with one of the big jumps on Snowdream can still give you goosebumps on a winter's evening.
C&C series as above.
Doom etc have got to be in with a shout for somehow creating such dank and brooding atmospheres with just an Adlib synth chip to play with.
Monkey island similarly for the sheer range produced under a similar leash.
And Tempest 2000, which was sort of Rez before Rez was ever thought of.
(again, there's so many that are missing here...)
Heh ... so I say I wasn't paying too much attention and couldn't think of many favourites then that happens. Mebbe it's a good idea I'm a coffee short of full service rpm at the moment.