Oh man, I'm glad I read through all the posts before I shared haha
M'aiq was always fun to find and talk to. I love his little jokes about what the developers addressed in the game.
Serana, despite being undead, always had more life to her to me than most other NPCs.
Sheogorath has to be my top - nostalgia and hilarity from Shivering Isles, and just as funny in Skyrim~
edit: Wanted to add Karliah because her voice is that of a goddess. She's also a very reasonable and honest (to a point) person. Rather refreshing in Skyrim in its current state.