Most definitely Final Fantasy XI
, seriously if there was one thing SE did right with the game it was the freaking sound track
, to just a list a few examples:
Anyone else who's played it knows those are far from the only good themes of the game XD.
Though aside from it I like most of the Armored Core's sound tracks as well; especially 2 and For Answer's sound track and the .hack games series has amazing OST too [both //GU and the original].