I'm a big fan of FF13's OST, it was probably my favourite thing about the game. I can't think of a single 'best' piece, but off the top of my head The Promise, Dust to Dust and Lightning's Theme are all great. The Sunleth Waterscape also has a way of sticking in my head :p

http://www.youtube.com/watch?v=XRf5XKeo-DQ
https://www.youtube.com/watch?v=Ckxy-i7Dc5U
http://www.youtube.com/watch?v=OOVPNVHW9FY

I agree with you. Lighting's Theme and Snow's Theme are superb. Solid OST, with its ups and downs, yet definetely some awesome songs!

The games that came straight to mind were:

Mass Effect 2 (the score that accompanies the suicide mission in particular)
Mass Effect 3 (not really epic so much as it is haunting)

Elder Scrolls: Morrowind, Oblivion & Skyrim (Jeremy Soule's various different tracks and remixes with each entry in the Elder Scrolls series just fit perfectly with the mood of each game)

Final Fantasy VI, VII, VIII, X (the tracks in each of these games really stick in my mind and every time I hear the soundtrack it immediately conjures up the emotions of each one)

Shadow of the Colossus (I could be doing housework and the OST would still make me feel like an hero in a epic odyssey)

Awesome selection!

I had never heard gravity rush OST and it's quite something. Great song!

Yea, I know. That's definitely one of my favorites. Gravity Rush is amazing.

This one surprised me too. Never expect this from such a small budget game.

http://www.youtube.com/watch?v=Wew8kC4tvNQ

I couldn't agree more.  There was another song in that game, Will to Fight, that was also a favorite of mine.  Desperate Struggle, Saber's Edge, and Blinded by Light are also fantasic pieces.

Twilight Over Thanalan - Nobuo Uematsu

https://www.youtube.com/watch?v=iFE6eR9jHIs

Beneath Bloodied Banners - Nobuo Uematsu

https://www.youtube.com/watch?v=WfcYRqKQM0g

Child of Light, as a whole, it's a joyous aesthetical experience. Its soundtrack fills the gorgeous canvas with nostalgia. 

Suikoden II and Shadow of the Colossus.