From someone that almost exclusively runs rally races, I like the '08 Impreza WRC car, the '03 Impreza rally car and the Super Evo the best cars. The '08 Impreza has stump pulling torque which is great for digging out of corners and uphills, the '03 Impreza makes great power throughout the powerband, and the Evo just has great grip. On all of those cars I run the stage 3 engine upgrade and the mid-rpm turbo, as it will give you the best torque.
I like the Ford Focus WRC '08 and Suzuki Swift WRC '08 cars too, but they lack a 6th gear, which really makes a difference, IMHO.