Megatron, for not shooting his overly trusted second-in-command in the head, repeatedly.

Still trying to decide whether StarScream was an Autobot provocateur, or just plain stupid. The good guys were seldom the reason Megatron's plans failed-- it was always StarScream's fault!

Mind, I'm referring to the cartoons. Be nice if someone made a live-action Transformers movie. Ow, I bit my tongue there...