OP, are you aloud to run any tire you want in the class you'll be Autocrossing in, like a DOT-R compound ? Or do you need to stay with an actual street tire.
If I had to choose a street tire, I second the Hankook RS-3 suggestion.
If money were no object and you could run a DOT-R spec tire, the absolute best bet would be the Hoosier A6.
Your second choice, and considerably cheaper would be the Hankook Ventus Z214.
BIMMERS FOR EVER !