The best autocross car is anyone that you can go and compete in:D
Every stock class has specific cars that are stong for a given class. It's probably a best bet to determine what you really want of a car, then make a decision from there. Classes will change and cars will change classes, so you'll have to factor that in.
One way to get a look at what the strongest cars in each class is to look at the back of the scca rules where past national champions are listed.
Parking Lot Hero
2008 WRX daily driver
2003 ZX3 23D Autox Car 42 FSP
"Hella Functional - Raced, Not Hard Parked"