Fairly simple premise, but hard to nail down since there are so many to choose from, but here are mine:
5.) The Elder Scrolls: Morrowind - Jeremy Soule could get this nod for any number of games (his work on both Guild Wars soundtracks is almost as good as his Elder Scrolls work), but I think Morrowind's soundtrack stands out as his most iconic output. But among RPG composers, he has no peer.
4.) Divinity: Original Sin - The saddest thing about D:OS2 is that it won't have the great Kirill Pokrovsky to produce it's soundtrack, as he sadly passed away within the last year. The fact that it is the only relatively new game on the list speaks to how great it is.
3.) Baldur's Gate - You can argue about whether BG2 is a better game as a whole, but I don't think there is any question that BG1 has the superior soundtrack. Wandering around the wilderness of the Sword Coast would have meant far less without Michael Hoenig's soundtrack.
2.) Diablo - There are really only 5 tracks in Diablo. The legendary Tristram theme of course, but also increasingly claustrophobic industrial nightmares as you went further into the dungeon. D2 is also stellar in the soundtrack department, but the original work Matt Uelmen did here is a masterpiece of economy and mood. Special praise for the "Catacombs" portion
1.) Might and Magic VI: The Mandate of Heaven - A more magical and mystical set of high fantasy music I have never heard, and likely won't. The soundtrack to M&M 6 is what adventuring in a fantasy world SHOULD sound like. Paul Romero and his team are well-known for their work on Heroes 3, and it is justly praised. But nothing holds a candle to walking around New Sorpigal the first time hearing the enchanting themes that, more than anything, set the mood for this game. Absolutely perfect.