Took me quite some time to figure out who to vote for.
Edmure Tully not really stupid he just wasnt informed of what the plans were and therefore acted as he felt was the smart move at the time.
The Slavers were not really stupid in the sense of the word stupid, they werre just tricked by Danny as they didnt know she spoke the language and therefore had the situation under their control.
Theon was actually smart, but didnt think his plan through, he had a plan perfected it to perfection and then he didnt think of what to do next, kind of like Bush
My vote goes to Robb, his stupidity came from inexperience. His marriage for love is a beautifull thing, but stupid thing to do, i think Robb didnt think of The Late Walder as a threat, only thought of him as a silly old man.
Karstark affair couldnt have ended in any other way, so no foul there.
And then there are all the little things, losing his only real bargaining chip againt the lannisters and so on, just to many small mistakes and they added up to what is called "The Weding Incident"