Comparing Presidents you don't like to Hitler has become such a cliche, people have been doing it for decades. And don't get me started on the interenet (something something Godwin's Law). Although I don't know if just changing the reference goes far enough. How about some creativity, to borrow a line from Troy from Community, "[President I don't like] is the AT&T of people." Have some fun with it.
AT&T is the Hank Williams Jr. of telephone companies.
Caligula seems like he'd be a strong candidate.
Also an option, "__________ is the opposite of Batman."
Yeah, but isn't superman kind of the opposite of batman.