To contribute to this thread, I would honestly in a heartbeat fight Rich Franklin. I believe deep in my soul that I could beat him straight up.
Also, call me crazy and he may not be UFC, but I would step in there with Fedor.
People I would not step in there with or at least without getting totally dominated, are as follows;
Anderson Silva: Because our styles are identical and he has been doing it much longer and I truely fear his limbs.
BJ Penn: Because the guy is just too goddamn good for words, his MMA record may not dictate it but he is such a force to be reckoned with in MMA.