Well, the games call him Eggman, every Sonic game I can think of they call him that. But the show (I'm 90% sure that's what your avatar is from) called him Robotnik. So it depends on who's looking at the avatar... someone who plays the game, or someone who's watched the show.
I always thought it was Robotnik, but Wikipedia says that his name is Eggman. And there's a character limit, so I can't put Eggman/Robotnik in the user title. Eh, it's only for a week. I'm sure you'll manage! :D