My favourite would have to be Gran Turismo 2. Next would be the original Gran Turismo. Even though i've got a copy of Gran Turismo 4, I can't play it since I don't have a memory card.
Gran Turismo 2 has excellent gameplay, although some may argue that it has a somewhat exessive amount of unnesscary vehicles. After all, who wants to race a Diahatshu Midget anyway? However, graphics have been left virtually unchanged from the original and sound is boring and repetitive. Boasting tons of tracks, rally driving, 500 cars, and race modifications, in terms of overall racing content and gameplay, Gran Turismo 2 is clearly the winner. The game is alot easier than the original Gran Turismo, particularly the liscense tests. Lacking in sound and overall innovation, Gran Turismo 2 may have been the victim of hurried development and the rush to get the game onto store shelves as quickly as possible.