What's outside the Mushroom Kingdom? Who lives there? There's never any information to establish what's being conquered elsewhere... for all you know he beat up 6 old ladies and took over their huts.
I can't choose! I mean they're both pretty evil and messed up Bowser has a thing for kidnapping blondes and some how spawns all these nasty children but Robotnik has a thing for kidnapping little furry animals and birds while making these robotic monstrosities.
I guess if I had to choose... I go with Robotnik and this is all on his voice actor, guy really nails the goofiness of the character.
Eggman was clearly the better villain. Bowser only had it out for the Mario Brothers and Princess Toadstool. My favorite would have to be Bowser though because I love his boss battles more and his character design so much more (especially in the early years).