Motor Sport Elise '99 is one of my faves. Don't need to do much to it, just a little bit of modification and tweaking.
Nissan GT-R and Skyline R34 Nur (Premium) - these are both pretty awesome fun
458 Italia is brilliant but I also quite like a fairly heavily modified 512BB, it makes a great noise and that 70s styling is rather nice to look at during replays, makes a great noise too.
I have the LFA and 670-SV Murci but whilst they are good, I don't think they're great....
SLS AMG is fab, love that too
M3 CSL with a few mods - gotta love low-weight M cars
If I'm in the mood for crazy oversteering, drifting craziness, I always go for the P4 race car or TVR Tuscan Speed 6 - excellent fun, but not always the quickest times
I like my closed-wheel Nissan R89C car too, one of my favourite periods of sports car racing and a very striking race car which tears your face clean off.... I've tried (in vain) to get it round the ring without crashing, not managed it yet, just tooooo fast
I could go on, forever..... there aren't many cars that I don't like frankly.