I don't mean the best music from a game at all. I mean YOUR favorite. The songs you immediately recognize and can listen to all the way threw with a smile on your face. Mines mega man 2 with mega man 3 a close second. Every time I hear those songs I can't help but think of ghost busters, hanging out with my cousins,are you afraid of the dark, dunkaroos, and of course how awesome it felt every time I beat a boss in one of those games because that shit was hard!!!!
I loved Outrun back in the day. The arcade music was so popular that my Spectrum version shipped with a cassette of the soundtrack from it. Xenon megablast was the best from my Atari/Amiga era. Or Gods.. Basically anything the bitmap brothers did!
My favorite in-game music is from two series. First : Goldeneye Second: Rainbow Six (original ) Still find them and listen every now and then.