There might (might, see later) be a set of values, but there definitely isn't anyone or anything that, given dictatorial powers, can be trusted in applying such values forever fairly without abusing or misusing their power.

I'm far from sure that even the rationalists' god-like AI could play that role, even in the extremely unlikely hypothesis that it will materialize at some point.

Not to mention that the best set of values could change with the circumstances, and most of all values are a vague concept, WAY too vague to make a dictator behave properly in all circumstances, even if he were somehow compelled to always follow them.

> humanity would be better off if the other political party was gone [and my political party had sole power]"

Even people wishing that don't necessarily long for a dictatorship, far from it; you could wish for the party you loathe to not exist, but still want everything else of your democratic system to stay unchanged (the possible parties are infinite, removing one doesn't necessarily turn your system into a single-party system).