Outrun 2 for me.
But the (sad) fact is that many of the racers released in the last generation are still better than the ones that have been released this gen IMO. I'd still rather be playing Daytona, Daytona 2, Ridge Racer IV, etc. than anything released right now. F-Zero GX and Outrun 2 have been the two best racers released so far though.
It's the sim racers that have been much improved, but I hate those games.