Every one of the Parasites deserves what karmic justice is meted out to them, but killing Doctor X somehow feels the most rewarding. Probably because he gets the most build-up.
Susan: I think it's time to find out how very mortal you are, Doctor...
Turning the tables on the Second and Third Parasites is awesome in a particularly Troperiffic way. All the B-Movie horror they've inflicted on Susan is paid back tenfold when she hunts Gladys through the mansion while wearing a rather disturbing gas-mask and totting a shotgun; the culmination of the sequence is incredibly satisfying.
Getting the ending where Mitzi lives. It's just so full of feelings that there are no words.
Susan overcoming her depression and self-loathing by defying their anthropomorphic representation, the Queen of Maggots, and refusing to blow out a candle. Made especially powerful by the voice actress's soft but conviction-filled delivery on: