Here's my list by generation:

8 Bit = Death Race on the Vic 20 (first racer I played to death (excuse the expression!)

16 Bit = Lotus Turbo Challenge II on the Amiga (I actually lost a girlfriend to this game!)

32 Bit = Sega Rally or Manx TT on the Saturn (Just couldn't believe these games with the wheel)

64 Bit = Roadsters on the N64 (I don't really know why, it was just so smooth compared to any PS1 racer I can think off.)

128 Bit = Burnout 2 (It's just the epitomy of refinement in the arcade racer genre - so slick, smooth and responsive it makes you feel like a real demon!!)

An honourary mention goes to F-Zero GX. Only honourary cos it doesn't feature 'real' vehicles. But htis game is just so amazing, it just pisses all over Wipeout for race thrills. Absolutely blinding!

If I had to pick just one though it would have to be Lotus II on the Miggy. It just ate years off my life!