Scale is a problem but at the same time, the bio for megatron says he took out fortress maximus. if there's a size disparity in a fight, that's got to be it.
I don't think scale makes a great difference, smaller characters can often make up for the difference in size with other advantages, speed and being a smaller target come to mind quickly.
My vote is for Rampage as well, he's batshit insane and has an immprtal spark, so kind of hard for him to lose.