I'll start by saying that I have Sport track, always remove 'traction' magnets, and don't time my laps!
I just got a Scalextric Audi R8, and I must say it's brilliant. Out of the 150 plus slot cars I've owned, I'd vote for this as the best handling of them all - not the fastest, or the one that sticks to the track best, but definitely the most fun to drive - it looks amazing, the mabuchi motor is just about perfect, and (a bit subjective this) it has what seems to me to be a very realistic handling style - it really looks like the real car on track.
Others I've owned that I'd say run the Audi pretty close (for similar reasons):
Fly Porsche 934
Fly Renault 5
Ninco Lamborghini Gallardo
SCX Ferrari FXX#
Slot-it Lancia LC2
Which of the cars you've owned would you give this accolade to? Which cars should I be looking out for to add to my collection of great looking, great handling cars?