That depends. Do you want an arcade "floor it and go" sort of game, or a hardcore racing simulator that'll give you new respect for professional drivers as the unforgivingly realistic driving physics punish you for every mistake?
For the latter, there's the likes of rFactor (tons of mods, so you'll probably find something you like), iRacing (requires a subscription, but is said to be currently the best hardcore sim at the moment), Richard Burns Rally (has the RSRBR mod to add tons of new cars and tracks, and is generally regarded as the best rally sim out right now), Grand Prix Legends (old Papyrus sim that goes back to the 1960s Grand Prix, and is also ruthlessly difficult), and the various SimBin titles like the GTR series.
At least, all of that's what the SimHQ folk are into. I should take another shot at RSRBR, but it's damn hard and I could use a better wheel and pedals.
For the former...I enjoy the Burnout series, but it's less straight-laced racing and more "smash up the competition", albeit not to a Carmageddon extent. Also, it's been years since I last played Need for Speed: High Stakes, but I feel like giving it another shot if I ever get the chance.