King K. Rool

I love his design and also find the idea of a pirate king really fun. I know the old Rareware site said that King K. Rool took the bananas to starve the Kongs so he could get their island, but I like the sillier take that he just REALLY likes bananas and refuses to let anyone get in his way.

Darth Nihilus

This character is a fun exploration of a Sith as an unknowable Eldritch force rather than an evil knight or lord. His hunger for power is so great that it has sapped even his physical form. By the time of KOTOR 2, he's just a spirit in a mask and armor. (Apparently his design was inspired by No-Face from Spirited Away.)

Doctor Eggman

He's an icon for a reason. He's such a proto-typical mad scientist and the idea of turning Sonic's friends into robots is so diabolical.