Without a doubt, in my opinion, JT's move firmly places him at the apex of stupidity.
Specially considering all of the other dangdumbest moves were usually precipitated by OTHER people.
Erik was swayed by Natalie's story.
Ian was berated by both Tom and Katie.
Even James keeping his idols was precipitated by his "trusted" alliancemates making him believe he didn't need to use them.
And in JT's defense, giving away an idol, SO FAR, has always resulted in a positive experience for the givee.
When Jenna gave her necklace to Heidi, she insured Christy got voted out, paving the way for Jenna to win.
When Todd gave his idol to James, as we know, it didn't work out for James, but Todd went on to win.
When Russell gave his idol to Parvati, while we don't know the final outcome, I dare say they both are headed for the finals.
The only time it really hasn't worked out was when Sugar used it on Matty. While it did allow for Sugar to make the finals, it didn't give her any credence with the jury.
Now, JT's big problem lies with the simple fact that his move is based in assumptions. He assumes Russell is on the outs, he assumes the ladies run the tribe. And as with all assumptions it does nothing but amplify the lack of knowledge of the assumer.
JT is ultimately the "stupidest" because he's made up a story to hang his strategy off of.