I know this might sound daft to folk today, but to me one of the all time greatest tunes for any computer game is the music in the old C64 game Forbidden Forest, at that time apart from the size of the characters in the game no-one had even heard such music coming from a home computer, totally blew me away that one did...
(PS:Loco on the C64 was also another brilliant piece of work...
On the Amiga side I'd have to say the music in The Faery Tale Adventure and the theme tune from UBI Softs ILyad are me all time faves...
