Personally there is no best but games like Gears, Payday, Halo, Dragon Age, Star Wars KOTOR, Crusader Kings (Though it can get grating after repetition), Dynasty Warriors, Fallout, Command and Conquer, Super Robot Wars, and Assassin's Creed tend to have good soundtracks.

Of course there's a lot of more but these are off the top of my head. The classics like Link and Mario still stand the test of time.