Probably an unpopular choice, but I´ll go with World of Warcraft.
I just recently got back into playing Classic WoW and immediately got hooked on the music again. From the mysterious flutes in the Night Elf territories, to orchestral music when you enter Stormwind, or the war drums in Orgrimmar...
Can´t speak for the newer expansions, but classic WoW had great and fitting soundtracks in every territory.
(I didn´t play Age of Conan enough to really remember its soundtrack or feel nostalgic about it, but I think it was a good one as well)