Cliche, but Final Fantasy VII is still one of my favorite soundtracks. Very little filler, very huge pieces.
Donkey Kong Country 2. David Wise, you genius.
Really loving Lost Odyssey's soundtrack too.
Mega Man 3

I could go on for eons, but those are my favorites that I could listen to at any time.

Not cliche at all, Nobuo is a genius as we all know. So yeah, definitely anything in the Final Fantasy series. III, VII, VIII, and X in particular. I also love Marty O'Donnell's stuff in the Halo series, I actually bought all the soundtracks when I was a kid. I'm a bit worried if the new composer's score for Halo 4 will live up to Marty's work. I also enjoyed the music in Fable 3. Besides original soundtracks, Forza usually has a good selection of songs.

Ya, the Halo 1 & 2 OST are great and innovative. "Halo" from Halo 1 is an obvious great piece. "Unforgotten" & "Heavy Price Paid" from Halo 2 are stand out pieces. I love the use of percussion on Halo 1 as well. Polyrhythmic goodness.