1. Streets of Rage 2 (Sega Megadrive/Genesis............. Yuzo Koshiro's hardcore techno beats have my foot tapping and my head nodding up and down everytime i play this game. quite simply the best music of the 16 bit era by a country mile in my eyes.

2. Shenmue 1 & 2 (Sega Dreamcast)........ as mentioned by Iveyboi above, even if you have not played Shenmue (and if you have not? WHY NOT?) the music is nothing short of epic, the sountrack is full of life and emotion.

3. Jet Set Radio Future (Xbox)...... one of the most bizarre soundtracks ever created, yet somehow if fits perfectly in to the wacky world of JSRF.

4. Tony Hawks Pro Skater 3 (PS2) Not only is this the best game in the THPS series, but its also the best soundtrack in the series too (although i have to say, a few other THPS games had superb soundtracks too)

5. Revenge of Shinobi (Sega Megadrive/Genesis) the boss musiuc alone is epic, but throw in great music for just about every other stage of the game, and you had a winner.

Honourable mentions..........

Need For Speed: Hot pursuit 2
Super Mario world
Various Sonic games (especially Sonic 3 & Knuckles, Sonic CD & Sonic Adventure 2)

And finally......... a new entry....... i just started playing Castlevania: Portrait of Ruin last week, and WOW, the music is sensational, and low and behold..... MR Streets of Rage Yuzo Koshiro composed 5 tracks for the game (and ironically its one of his tracks that is my fave in the game, "invitation of a crazed moon" superb game, superb music)

I have five favourites I'll give my favourite of each track of each ost...

Skies of Arcadia  (Little Jack Ship) http://www.youtube.com/watch?v=LFcH84oNU6s

Chrono Trigger (Frog's Theme) http://www.youtube.com/watch?v=fMzkUriqXvs

999: 9 Doors, 9 Persons, 9 hours (Morphogenetic Sorrow) http://www.youtube.com/watch?v=WjNi14fKSow

Lost Odyssey (Neverending Journey) http://www.youtube.com/watch?v=135Gltfkxeo

We Love Katamari (Kuru Kuru Rock) http://www.youtube.com/watch?v=fn59_HopFRc&feature=related

Halo franchise but specially Halo 3: ODST. Halo 3: ODST Music is so quiet, is so sad, is Magic!!!

Sometimes my favourite games are determined by music, but my 2 fave games of all time (FF6 and Chrono Trigger) could not match the awesomeness that is Final Fantasy 4 for music. It had everything, including one of the best (if not THE best) last boss battle music ever!

Others include Secret of Mana, Skies of Arcadia and Tales of Symphonia

No particular order here.

Super Castlevania IV (Beginning/Treasury Room)
Donkey Kong Country 2 (Stickerbrush Symphony/Kaptain K. Rool)
Mega Man X (Launch Octopus/Central Highway)
Mega Man 3 (Spark Man/Shadow Man)
Mega Man X2 (Wheel Gator/Counter Hunter Stage 1)
The Legend of Zelda: Majora's Mask (Giants' Theme/Clock Town [3rd Day])
The Legend of Zelda: Twilight Princess (Stallord/Diababa [2nd Half])
Metal Slug 5 (Final Attack/Fierce Battle)
Shadow the Hedgehog (Digital Circuit/Cosmic Fall)
Kirby Super Star (Run, Kirby, Run/Dedede's Theme)

I've never played DKC2, Majora's Mask or the any of the Mega Man games on this list, and Metal Slug 5 and Shadow the Hedgehog are awful in every other regard, but I've listened the living rabbitshit out of their soundtracks.

Does Guitar Hero count? :D

Just kidding, here's my top 5:

1. F-Zero X/GX (badass straight through)
2. Metroid Prime series (adds perfectly to the atmosphere)
3. Mario Kart 64 (Rainbow Road and credits music never gets old) :')
4. Pokémon Blue/Red (obvious reasons)
5. Halo 2 (not a big fan of the changes from Halo: CE, but the music beats any of the others)

There are so much amazing music in games! They are art!

But Whole Halo franchise has great music with feeling! There are so many unforgettable songs.
Another is Alan Wake. Petri Alanko just knows how to write music.
Mass effect has also very memoriable soundtracks.

This gen, probably Nier or Twilight Princess.

1. Shenmue (Dreamcast), I own the official soundtrack

2. Halo Series (Xbox, Xbox 360) I own several official soundtracks as well as the new vinyl record

3. Panzer Dragoon Saga (Saturn), I own two official soundtracks

4. Mass Effect Series (Xbox 360, PC, PS3)

5. Metal Gear Solid: Twin Snakes (Gamecube) I own the official soundtrack

6. Headhunter (Dreamcast, PS2)

7. Chrono Cross (PSX) I won the official soundtrack

8. Zelda Series (N64)

9. Monkey Island series (PC)

10. Portal 1 & 2 (Xbox 360, PC, PS3)