rFactorCentral.com has all the extras. This list of tracks, cars and race series is never ending... Almost ever kind of car and track you can think of is done or in progress. This is not a game but a SIM. It takes a while to get used to driving htme as you need ot set up the cars and your equipment. Must have a good wheel and pedals to drive them.
Dunno about "Best", but I really enjoyed TOCA2
Its a bit old now, and the cars oversteered A LOT, but it was really fun. I played enough that I could probably run many of the UK tracks and feel right at home..
Got it about a month ago and have been playing with it. I really like the ability to add on to it easy. Plus it comes right out of the download with 4 types of racing. Open wheel, Car cars, rally and Nascar. The Aussie V8 add on is kickin!