Personally, I Like the: "Sherbert Pickup Hot Rod" "Walter Hot Rod" and the "Walter Rocket"
People say the Walter Rocket is the fastest but i think the "Sherbert Pickup Hot Rod" is the fastest out of all, It has great acceleration and great speed, And it looks crazy cool, + you get an achievement for getting into it
The cars are not in any particular spot, But to find them, Go to ally parking lots, Behind buildings, and if you get lucky, you will see them parked on the side of the road or driving
I have only seen these cars on the right half of the map
Personally got the achievement with the Ascot something. One of the two cars sitting outside of the house
SPOILER - - - - - - - - of the old man from prison who had the hit out for him that you save. Actually got it as I was driving him to the train station or air port or wherever the hell I had to take him.