I've been using the Hoonicorn and Twerkstallion, recently....if you have the Hoonigan car pack. Otherwise, I really have used the Holden Torana HE that was modded to have big power, torque and AWD. If I need to win a race with style points (in the Hot Wheels DLC) I usually use my Corvette Z06 HE with light mods.
In the leaderboards, I also see the 97 Mazda RX-7 being used quite frequently, which you can get for free through a midnight battle, I think...