I'll start with something that many years ago completely disarmed me with its sheer beauty, uniqueness and atmosphere.
This comes from Matt Uelmen, and back it the day it was a shock that one could create such a great, dark and gritty hybrid of classic medieval fantasy with modern sound, using haunting samples and overdriven electric guitars.

Diablo I maybe wasn't an RPG. But I've spent countless hours with my friends playing it, almost every day after it was released. We even carried our heavy computers to each other's houses, with those old-fashioned CRT monitors - man, that was some exercise. But it was also the only way to play Diablo on LAN, and almost nobody had Internet back then. And it was a great experience, killing the Butcher and skeleton king Leoric countless times, reaching catacombs and then Diablo in hell.

Later there was an expansion adding the "subterranean jungle" levels and some more dark corridors leading to Na'Krul (which *somehow* I've killed with exactly 2 fireballs! Had it recorded on tape to prove). But it wasn't the same without such a great soundtrack that was created originally.

In my opinion, Diablo II and Diablo III never reached this level of musical craftsmanship that helped to make the first game such a huge hit. Heck, almost no other game since then could compare, in their whole history. That says something.

There are other gaming soundtracks that I love for different reasons, so the above may not be necessarily my number one, but it's certainly in top 10.

And what is your favorite soundtrack?

Is it from a new game like The Witcher or Elder Scrolls: Skyrim, some classic oldschool thing like Planescape: Torment, Baldur's Gate, Heroes of Might & Magic III?
Or maybe it was some retro, SNES, Amiga title? What would you recommend for an RPG gaming session?