[Kotohono]

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:

Spoiler for The Ruler Of The Skies; Bahamut's theme:

Spoiler for Fighters of the Crystal; theme of the Ark Angels:

Spoiler for Stargazing; Windurst Shadow Reign:

Spoiler for Iron Colossus; Alexander's theme:

Spoiler for Ode of Life Bestowing; A Cristalline Prophecy:

Spoiler for A Realm of Emptiness; Promathia's theme:

Spoiler for Ragnarok; Odin's Theme:

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].