So far, I've only tried Rome and Grand Valley, but the best cars for me were:
Rome:
Spoon CR-Z, 371PP with SH: easy win with such low PP
Toyota MR-2 1600 G-Limited Super Charger '86, 387PP, SH: quite agile, easy win
Lancia Delta HF Integrale Evoluzione '91, 397PP, SH: needs about a mile to break but will destroy the field on acceleration
Audi TT Coupe 1.8T Quattro '00, 398PP, SH: the reasonable, easy choice
Alpine A110 1600S '73, 409PP, SH: my favourite. tail-happy but easily controllable
Lancia Stratos, 406PP, SH: the "honestly I didn't do that on purpose" choice. ****in' hell, would be an easy win if for once I would be able to keep it on the road for longer than one lap
Grand Valley:
Spoon CR-Z: not powerful enough for me, the top speed just isn't high enough
Lancia Delta & Audi TT: good choices, easy win
Alpine A110: more of a handful than in Rome, still easily winable
Stratos: still trying to stay on the road