I chose Dalek, because it's the easiest choice. That episode was touching and horrifying at the same time, and I never knew I wanted that combo to exist. BUT. I really liked the mystery around that capsule in Army of Ghosts/Doomsday, and the Cult of Skaro emerging from it is a great scene, in my opinion. This episode also has one of the funniest scenes, with Daleks sassing the Cybermen.
Daleks in Manhattan and Bad Wolf two-parters were good, but they didn't blow my mind. And I feel like The Stolen Earth's main focus wasn't actually on the Daleks, but on Doctor's relationship with his companions and his history with Davros.