I used to think GT was the best, and was always planning on buying anything Sony decided to launch GT on...
Then I built my PC, and found GTR.
Suffice to say, I haven't touched my PS2 in a few months, and the last two games I remember playing were Family Guy (awesome, lol), and The Getaway: Black Monday (brilliant game)
GT will probably never make it to the PC, since Sony is selling their consoles by making it exclusive, but if they did release GT4 onto PC, that would be nice. It still looks good, and would likely have great modding potential, without Sony losing out PS3 sales, by giving us the older game instead of the newer GT5/GT HD or w/e the hell they decided to name it now.
What I would really like is a racing game, that's a simulation, but you can either choose to be a street racer, or a circuit racer with big contracts and stuff.
Having a whole island, like in Test Drive Unlimited, a few large and small professional circuits, and hundreds of miles of various roads, from bumpy C-roads with plenty of twists, to long straight, smooth motorways, to get around the island. Greater customisation options, on a par with ProStreet (which seems to have the most customisation options of most racers I've seen), with the chance to do precision settings, like in Simbin's racers.
Gentlemen, I think we all agree, that would make for a pretty sweet game.