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.
Mods: Header, CAI, Shark, Koni Sport Struts, Special Lowering Springs, Rob43 9 Way Shocks, 3.38 Diff, 75 Shot Of N20
E46 M3's are just tasty snacks i eat for lunch.