I'm pretty surprised no one has mentioned the Focus RS as It's generally allowed in road car rooms from what Iv'e seen. It's poor when it comes to grids start's but guaranteed will eventually pick apart the competition after a few laps on a bendy track such as Brands or Ascari, providing there's no concept cars and such, just have to be patient with the massive understeer that comes with it. The only car's that I know of that would stand a winners chance against it would be one of the RX-8's, forget which one, the ZZ, the Ferrari GTO and maybe the NSX tyre R.
For tracks with long stretches of road and high speeds like Spa or Monza I'd go with something like an RX-7 or something that packs a little more punch, but then again the Focus RS would never be a bad choice in my opinion, so long as you want to purely win races and don't mind being looked at as a newbie every now and then