WRX Sti '10 hands down. At Eiger Nordwand Short Track on 500PP with Racing Softs, i can post under 1.03's-1.04's with 500PP (Unsure about sports tyres, but it would still be the best), however i do race this car more at Eiger with 550PP, in which i have posted a 1.01:370 with Racing Soft tyres. The cars low speed acceleration is the best in the game for 500-550PP races. It beats pretty much everything off the line, and it's exit speed out of the tight first few corners is just immaculate.
@mr.wigglesaur: It does have a touch more understeer yes, but it makes up for that with exit speed. Simple fact is, tune the '10 like i have, and the understeer is seriously reduced.