Above: the original GTA, back when it hadn't yet even seen a Playstation release. Gotta love a crazy track like this to pump you up while being chased by the cops and slaughtering innocents!
I don't know if this counts as its main theme, but you hear it the most often as it's the Town music, so everyone remembers it best. Well, it's also remembered because it's fucking awesome, of course. It complements the atmosphere of Diablo too well...