No one is mentioning Metal Gear Solid or Dark Souls/Bloodborne, I'm surprised. They have my favourite soundtracks.

I could mention a ton of games with great sound tracks. One game series with some supurb music to fit the mood is Monster Hunter. Don't let the name fool you: the name is generic, but the series is epic.

Just the opening scene of one fight, and knowing that i'm going to be taking this thing on myself is what got me interested in the series.

The actual song in full

The in game song, which sounds comparable to the orchestra version

The Moving Mountain of Sanctity Massive dragon, which i turned into a custom ringtone for when my girlfriend's mother calls me.

Reincarnation of Light and Darkness AKA SHAGGY

Not everything is massive, and some things are cool, despite being "small."

Tigrex and Nargacuga Google those names.

A Ghost of a Deep Forest - Chameleos Giant Chameleon Dragon

Big cats

Monster Hunter Tri (wii) Theme

Town themes don't suck, either

And house cats

Anything by Jeremy Soule is amazing. I especially like the Morrowind soundtrack.

Don't know if Its quite my favorite, but I listen to the Dear Esther soundtrack way more than any other video game soundtrack.

I cant really say what the worst soundtracks are. If its that bad, I just mute it, and promptly forget about it.

I found the soundtracks in Final Fantasy: Mystic Quest and Gothic 3 so good, they made me overlook some of the bad parts of those games.

The worst soundtracks, I'm not sure, do crappy mobile games that use a music loop count?

I'm surprised nobody has mentioned Tropico yet. Let me be the first to do so then:
/game/tropico_reloaded
(The original one, not the later installments. "Buenos Noches", "Cafe", "Coco Y Ron" and "Huepa, Huepa" all come recommended.)

Samorost 3 (and also 2) has some great music in-game but most of it is not part of the soundtrack:
/game/samorost_3_cosmic_edition

Among some classic games, I have to mention Redneck Rampage (including the sequel):
/game/redneck_rampage_collection

And Jazz Jackrabbit:
/game/jazz_jackrabbit_collection
(Not sure the soundtrack is included in the GoG version.)

Out of the really oldschool games, I'm particularly fond of Goblins (Quest) 3:
/game/gobliiins_pack
(In particular, "Blount's Adventures" or "Were-Blount". I even used the latter as a ringtone.)

First-time poster, so can't include links. Sorry about that. Will edit my post when I'm allowed to link directly.