I thought Cro Cop was personally the toughest fight dos Santos has had since he joined the UFC.
If we're talking about Nelson and Carwin though, then probably Nelson. Nelson at least made the last two rounds somewhat competitive. Outside of one left hook and a nice sub attempt at the end, Carwin was a punching bag for three rounds.
Faber - Franklin - Velasquez - Hunt - Machida - Condit
Credit to M.C. for the sig!