GranTurismo 3 would be my favorite because of how big it is. I don't think I'll ever get 100%, but I know that there is always some race I havent beat yet. It has many options to customize your car's performance.
Can't wait till GT4 though. :D

Gran Turismo 3 is my fav hands down, I also like NFSU, I'd also say that Nascar Thunder 2004 is pretty sweet, can't beat the career mode.

My current favorite if Burn Out 2. I can't wait for Burnout 3 to come out. Midnight Club II was lots of fun also. I can't stand simulation driving games like Gran Turismo.

Grand Prix Legends. The best, bar none.

I've totally changed my mind after playing NFSU for a few days. I'm seriously hooked. It must be the Gran Turismo style race & tune action.