Prime
Okay, I admit, I haven't gotten around to watching the first TRANSFORMERS that had Walburg in it.... but when did Optimus become a bad guy? [Please, no one say he's lived long enough to see himself become the villain, that's just too sad.] I mean after what he went through with Sentinel Prime, I would think going against humanity and against Bumble would be the last things he'd ever consider doing, let alone actually do. I mean did someone mess with his wiring or his All-Spark when I wasn't looking?
"There is still hope." - Arwen