Saw this thread a while ago, finally decided to respond.
Racing car
Of all of the cars I could pick, it has to be the legendary Chaparral 2J. I remember getting so angry as a kid trying to beat the thing in GT4, and when I saved up to buy one, it won me over. It's a blast to drive, and it is freakishly fast on the track (In fact, it beat a Red Bull Jr on the Nürburgring Nordschleife). With a turbo, it's even scarier on high speed courses like SSRX. The car is amazing in my opinion, and it's become my goal to go to Midland, Texas to see the real steel 2J.
Road/Tuner car
This is a hard decision for me, but I find the Midget II to be so silly and fun to drive. In all honesty, it's a tie between the Opera S2000 and the Daihatsu Midget.
Concept car
Hands down, the Suzuki GSX-R/4. It sound great and is a semi-premium car (Interior is detailed), and is my fastest 500pp car on technical courses. Second place is the Plymouth XNR because it's just bizarre.