stupidest !
the word is stupidest, as in smartest, quickest, slowest, the "est" suffix indicates the superlative (or "most").
the suffix "ist" denotes a practitioner, follower etc of an activity, trade, belief.
thus we have dentist, socialist, flautist, violinist, plagiarist, egotist,
so perhaps stupidist is one aspiring to stupidity
sorry folks, I gave up waiting for the spelling police on this.