Am surprised to see such varied answers in this thread. I saw the title and thought theres no point looking in as the answers are odvious, i was wrong.
Well each to there own but here is my list.
Richard Burns rally = The best driver on the PC bar none although its rock hard and not for the faint hearted. Most will merely be thrown into the trees repeatedly but the skilled will know there is no better driver on the PC.
GT Legends = Best online racer. I care not about awards this is My opinion.
GTL has the most active pits on and the best overall feel of a racer online.
Races run well with minimal bugz and are huge amounts of fun.
Gent style racing is coming to the fore now with regular racers setting the standards. We stick together and join the same pits and vote idiots off sharpish.
RFactor = A terrific racer with many mods to suit all. For me the Superkarts are second to none. The speed of those things literally makes your teeth rattle.
Excellent fun and growing and developing all the time with recent mods such as the Terrific Dune Buggy mod by friends of mine The MrMen mod crew.
Big shout goes out to skills of MrIngall and [TSR]Raven.
Also the F3 Euro series on RF is brilliant the Nippons are great the Porsche carrera cup i could go on and on.
New tracks are coming out all the time and actual game players are developing the mods/tracks/skins etc.
The PC sim racing scene has never been in better shape to be honest.
It makes console racing games seem dated and childish and i don't say that lightly. My brother brought his old PS2 around when he got GT4 it was laughable. Now he has a 360 and PGR really there is No comparison at all.
With current systems and say a Momo like mine with graduating pedals and FF etc PC drivers are a world infront of console racing games.