I have a preference for Eggman, but I refer to Robotniks as Robotniks and Eggmans as Eggmans. I have to admit I rage a little when I see people harshly criticising the name 'Eggman' because it suddenly means he can't be taken seriously. Robotnik is totally sinister, right? Robot+nik. Do you know what that makes me think of? The name Nick, which in turn leads me to think of Phoenix Wright, causing my mind to settle onto Detective Gumshoe who isn't intimidating at all. So there, case and point, 'Robotnik' is no more goofy and unintimidating than 'Eggman'.
See? I can argue that one name is lesser based on a completely arbitrary set of rules, too. ¬¬
...Not saying anyone here does that. It just burns my poffins grinds my gears in general.