GTA V.C. & San Andreas. you know those guys spent good money licensing real music just for added realism of the era the games are set in. plus the commercials/talk stations are f***ing funny.
to tell you the truth I havent played enough games to know which is the best but I know a few that Ive liked...
for real songs I liked gta:vc and besides that I really was impressed by the in menu music in Civ (which ever is the newest one - I dont play many games as you can tell lol)
I think that the best game soundtrack would have to be Grand Theft Auto: Vice City. It is si much fun driving around on the motorcycle listening to that music!