Well, if by "stupidest" you mean "hardest" or "most difficult", then it's definitely the blue winged ironknuckles in the Great Palace of AoL. According to the site walkthrough here, they're called "Fokkas", which describes what I think of them very well. So hard to evade (they even follow you through high steps which you have to create by slashing blocks
while trying to evade their missiles) and nearly impossible to kill without getting killed first. Oh dear, how I hated those "Fokkas".
If you don't let AoL count, then I think it's either blue wizzrobes or darknuts in LoZ, but it always depends on enemy combination and environment. A single blue darknut or wizzrobe in an otherwise empty room is easy to get rid of or to dodge, but a room filled with several of each of them plus bubbles that disable your sword
and blocks in your way so that you can't even find a way around them or the wizard beams can easily kill you (This is the very room I'm thinking of, in Level 9:
. OK, no darknuts here but you get the idea)
If by "stupid" you mean "having low intelligence", then... maybe the Chu-Chus in MM. The only thing they do is following you very slowly, and that only if you get relatively close to them, and the damage they do is neglectable. But after all those usually aren't real "enemies" but rather some sort of "refill stations".
