I would have to say my favorite game soundtracks are in a Mexican stand-off between the Fallout series, Bravely Default series and the Fire emblem series, I couldn't pick a game out of the series because most of the games actually share a big chunk of the same sound track.
Megaman X series Dragon Ball Z Tenkaichi 3 (Dragon Ball Z games in general have good music but DBZT3 has the best) Brave Fencer Musashi Metal Gear Rising Revengeance (RULES OF NATURE) Sonic games Mario games
Although my current favourite one right now is easily this:http://www.youtube.com/watch?v=pNkQMtZAMAw
Plus the game is fucking brilliant, being a classic Doom fan and all.
http://www.youtube.com/watch?v=M3hFN8UrBPw
Dragon Ball Z Tenkaichi 3 (Dragon Ball Z games in general have good music but DBZT3 has the best)
Brave Fencer Musashi
Metal Gear Rising Revengeance (RULES OF NATURE)
Sonic games
Mario games