I'm almost always Bill Cipher, because it drew a lot of attention and conversation around 2 years ago when I first used him. And, he works. Not much else of a story about him.
Other than Bill, I have a couple other characters or so that I use on Discord and here, most notably Monika (from Doki Doki) and Nihilego. Those two I have also sold my soul to are a neat character whom I find interesting (Monika) and a creepy jellyfish that interests me psychologically.
If I'm not any of those guys, it's usually a character from a game/anime/etc. that I recently finished/am obsessed with at the moment and I liked the character and wish to spread knowledge of the work.