The Final Fantasy and Kingdom Hearts series is a given for me. The orchestra themes and the couple of more modern songs were pretty damn awesome.
The .hack//G.U. series has some good field themes, and I absolutely love the intro of vol 3
As well as Persona 3; got some cool pop and jazz kind of tunes I'm diggin' when I normally not into that style of music O_o
Star Ocean 3 had some interesting retro-ish style rock-ish game music ^^
Musashi the Lengedary Swordsman had some decent stuff too; not sure how to describe it, but it's like soft-core tech music I guess.
There's more game music I like I'm sure, but those were the ones that came to mind right away.