599 is fast. I've tried it on Daytona, I think it could be faster than the GTR but I haven't really tested to compare. I've stopped driving it lately.
I prefer these four fast cars in the game.
R34 GTR - strong, stable, powerfull. Intimidating. In professional mode, its easy to drive fast.
F430 - Best looking of the bunch and fast! In professional mode, its carves corners like a sharp blade, balanced around the corner when it starts sliding, but if you are not careful it can bite you.
R8 - stable at high speed and cornering. In professional mode, it's easiest of these four to drive.
Vette - a beast. Lots of power and engine grawl. In professional mode, it puts down a lot of power and can easily bring the rear end around on command from your throttle input.