Oh Christ, too much to even mention. Some nice selections here though. I have this thing where I prefer the music in the game, so if I want to enjoy a piece I must play! It's a good way to get back into old games. Many c64 games in particular have some of the best music ever.
Mostly old Amiga games: Battle Squadron, Menace, Leander, Shadow of the Beast, Unreal, Ork, Lionheart, Turrican, Traps 'n Treasures, Secret of Monkey Island, and the list goes on and on...
You can download the tunes in their original format from here: http://www.exotica.org.uk/
But you'll need a good player like UADE that can handle all the different module formats. There's also a bunch or modern remixes in MP3 or OGG...
I trully miss playing these games on the real hardware. Emulation just doesn't do justice, esp. with the more advanced games like Lionheart. Might have to spring for a Minimig someday...
There's just too many to list for me. It would probably include music from games that I didn't even like otherwise. Suffice it to say I feel game soundtracks have never been given the credit they deserve in their own right, with rare exceptions.
I'm with DoomUK really - I've loved video game music for so long and I'm still constantly discovering great soundtracks as old as the 80's, from greats like Jesper Kyd, Rob Hubbard, Hitoshi Sakimoto, Tim Follin, Kris Hatlelid, Dan Froelich, Andrew Sega, Alexander Brandon...the list goes on, and on...
It's pretty impossible to name even a few best ones, let alone the best video game song. But...this one sounds great, fits the place it's used in the game and it can be left looping on the background while drowsing.
The official "super arrangement" has a totally different vibe to it, but is definitely one of my favourite video game remixes/arrangements ever.
Kirby's Adventure on the NES has a ton of awesome music...and lets not forget about Sunsoft games: Batman, Blaster Master, Journey to Silius...
Just listing, posting youtube links would take all day. In no particular order:
Doom 1/2, Hexen 1/2, Duke 3D
Diablo 1/2, can't say about D3 yet
Morrowind (I found Oblivion's score to be utterly forgettable)
Warcraft 2, 3 has some good tracks but there aren't much in total
Vanilla WoW and SOME TBC music. I found WOTLK/Cata to have very repetitive and overdone music.
Just a single track from Mass Effect 1/2: Galaxy Map; the rest I found forgettable.
Serious Sam, also does ambient/action transition well unlike a certain Oblivion
Star Fox and Star Fox 64, though SF64 is held back by the weak MIDI
Legend of Zelda series
Fallout 1/2 and NV. Sorry to sound like a hipster yet again, but 3's soundtrack was forgettable to me.
As for composers/authors, my favorite has to be Junichi Masuda of Pokemon main series fame. Honorable mentions go to Matt Uelmen for the very memorable soundtracks of D1/D2, the people who made vanilla WoW's soundtrack, and Koji Kondo for the sheer quantity and quality.
That's just some of them, I'm sure I forgot something.