I don't know if I have an absolute favorite yet. Here are my takes on some of my favorites though...
SVT Raptor: When tuned, this it's pretty fun to have a pickup that can pull like it does. I kept the stock engine in mine, I didn't like the GT500 swap.
GT500/Shelby 1000: I lumped these together because they are so similar. I think I like the Shelby 1000 a little bit more because it has slight better traction stock then a GT500 tuned to the same power level. Both are comically ridiculous when tuned.
Veyron SS: This thing is cartoonish fast, and tuning it only makes it more ridiculous. I find myself cruising in 4th gear or higher because 1-3 gears are so crazy.