Fedor without a doubt has been better tested against opponents then Lesnar. There is no doubt about that. Frank Mir probably was Lesnar's biggest test, in either of their fights. Couture was coming off that huge hiatus and probably shouldn't have been thrown in there with Lesnar so soon, but it's not like Couture bounced back after losing because now he is at LHW, so that is debatable if he wasn't ready or HW was just too much for him anymore. I wouldn't say Sylvia and Arlovski were really at their best at the time they fought Fedor. Affliction hyped the hell out of Arlovski just to make people believe he had a chance against Fedor. Arlovski was still top 10 when he fought Fedor, but that still didn't mean he stood a chance against him. The Tim Sylvia fight was a joke. No way Sylvia even deserved that fight. He was coming off a Decision with Vera followed by a loss to Nog, he only got that for name value, I was never a Tim Sylvia fan, he just wasn't that great of a fighter. I give Rogers credit for pushing Fedor that far into the fight. Last man to push Fedor into Round 2 was Coleman in 2006, so Rogers did bring alot to the table, but still Fedor's skills were just too much. Some say Lesnar's biggest advantage over Fedor would be his size, but that proves nothing because Rogers is just as big as Lesnar height and weight.
Lesnar really hasn't been tested at all except for Mir, if you can call that a test because Mir literally got no offense in on either of their fights. Lesnar jacked him up with hammer fists in the first fight before falling into that kneebar. Their UFC 100 fight was basically all Lesnar. Mir would definitely propose a bigger challenge for Lesnar now that he has put on all that muscle.