My first favourite was my Audi R10. It was my first big purchase, and I still trot it out for night time runs at La Sarthe, which is why I got it in the first place.
Second fav would be the blue RE Amemiya RX-7, because it was a shock how well it handled, and how easily you can drive 10/10ths with it.
Both of those got eclipsed a couple of days ago when I bought a 787B for my a-spec Le Mans attempt. My God this car is near perfect. More than fast enough when you gear it correctly, sublime handling, and a motor that not only sounds fantastic, but revs like no other Group C or LMP car. I have it geared so that I am storming through the Porsche curves at the tip of third just on the edge of grip rolling the throttle to rotate the nose through the turns and it is epic fun.