I'd say 350z simply because it makes it's power across a wider ranger so you don't have to work as hard to keep the motor on boil. Although I'd be willing to bet an STi would do very well in autocross as well due to the flat torque curve (check out the way it makes it's power compared to the others, http://www.ultimate-racing.com/Produ...Dyno-Oct2.jpg). The newer S2k got a bump in displacement (to 2.2 liters), but I don't know how well it'd do on a track against the cars mentioned.

As for zipping around mountains I'd pick an Evo simply because I've heard great things about the over all driving feel of an Evo when compared to the STi and simply because a turbocharged engine is less effected by changes in altitude.

Although if your looking for a cheap autocross car, the AE86 has been known to do quite well in autocross events..