If we're talking about stupid as in hilariously entertaining, then mine would be this scene from Dragon Ball Z, or as I call it, the Most Massive Face Fault Ever. And that's about the most positive thing I'll probably ever say about anything that's ever come out of Shonen Jump.

And if we're talking stupid as in utterly retarded and which the writer should feel ashamed for even concieving...eh probably Naruto defeating Kiba by accidentally farting in his face. Actually, no, that wasn't the stupidest part. The stupidest part was how everyone treated that victory as if Naruto was actually this super-skilled badass, when he spent the fight largely getting his shit kicked in and only won out of dumb luck. I'm sure there are worse things in Naruto, but there's a reason I stopped reading that series early on.