My pick, would have to be that Spec Ed Optimus that came in the black box. The stupid spinning gimmick makes him always want to spin around when he is moved. I've had to blue tack his back to hold him in place.

Also the Megatron that came out in the black box as well seemed really tacky to me, things on him want to keep popping off if I touch it, like the tank barrel cover and such. I should have left them in the boxes.