Strange that for all the (reserved) criticism of GTA IV's exaggerated bodyroll no one has cared to mention Killatomate's Realistic Driving and Flying mod.
It's what turned my appreciation of the game's vehicle physics from "promising but awkward" to a definite "hell yeah" and that while using the less than ideal driving input method of keyboard and mouse.
The only downside to what I consider a virtually mandatory install is that it's somewhat ruined the 3D era GTA's for me when it comes to the "A" part.
I don't know if I have a favourite car; initially, I'd mostly drive the Coquette, but that was more because of the A-Team flashback than its zippy nature.
Enrique has been delivering more than a few Buffalos, a very useful vehicle due to its speed, easy control and sturdiness.
My vote in the sports cars category goes to the Super GT/F620 - very fast and nice to drive, while looking a lot more stylish than the typical speed demons.
Then again, most recently I've been enjoying cruising around Algonquin in a beat up Sabre, so as far as I'm concerned "it's all good".