Need For Speed: Carbon was the most impressing racing game I've played up until this point. Nothing can beat it for me, even though games nowadays are really well-done.
To be honest, I really love this mobile racing game Hill Climb Racing, it is interesting IMO, and I can be playing it long hours, and the time goes on so fastly while playing this game.
If the GTA franchise doesn't count as racing, then I guess Rockstar's Midnight Club games are my favorites. I find racetracks boring so street racing games where you drive through a city are more interesting to me.