By dumbest I mean so stupid that you feel embarassed about getting turned on by/getting off to it, something that no other sane human being would find remotely amourous. Something that anyone else or even you yourself looking back find hilarious that you got aroused by it.
Here is mine: Machoke's sprite from Pokémon Yellow because the artistic positioning of his elbow hiding his crotch implied that he was naked. I took my GBA SP to the bathroom with Pokémon Yellow in it, looked Machoke up in the Pokédex and...you know the rest. I was a very deprived child.
Your turn.
Updated by Peekaboo