Hitler, Stalin, Nazis and Communists should be off the table (and I'm even caving a little on Socialist - because Western European-style Social Programs are, in my opinion, perfectly compatible with Democracy - while the others aren't. Sadly, people don't really know what is socialist and is not, so they misuse it quite frequently, but I'll still leave it on the table for now).
Mass stereotyping doesn't do our politics any good at all. But calling Bush or Obama "Hitler" still isn't as callous as calling 70 dead kids "Hitler Youth". I find that one beyond the pale.