I just completed this last night, and I tried several options that I had in my garage to find out which I liked best. You'll get several opinions on this question. I tried the M3 GTR Race Car, Abt Audi, Mercedes 190E, and the Alfa 155. I like BMWs, so I was hoping I'd like the M3 GTR Race Car the best, but I didn't. All the FRs were a bit too over-steery for me, and I drove best with the 4WD Alfa, even though it was lower in HP. The 190E was the least over-steery of the FRs.
But, they're all great cars and fun to drive. I don't know which are currently available in the OCD or in your UCD, but you can't really go wrong with any of the options.