For me, of all the Arrancars that Ichigo have fought, Ulquiorra has been the opponent whom he wasn't been able to defeat twice and the opponent who finished him off without further adue.

During their first encounter at the Karakura Town, Ulquiorra simply analized Ichigo while fighting Yammy Llargo; claiming Ichigo as a piece of trash and unworthy of being killed because of Ichigo's current level and unstable reiatsu, that it will just destroy him in a long run. At the very start, Ulquiorra has been provoking Ichigo and at this rate, it's a hopeless moment for Ichigo to defeat the Arrancars, but gave him the will to step forward.