I went with Megatron, simply from the toon categorization of the Poll.

Megatron had a pretty steady personality of solid evil from comic to toon and back again.

Though I would put CC a close second, he was quite what I expected of him in the comic, but somewhat flip-flopping in the toon. One moment hatching an evil scheme to wreak havoc, the next minute crying like a schoolgirl and begging Destro to save him. Instead of cowardly interactions, they could have had CC saying things like: "You will regret this day," or "I will have my revenge."

Of course, killing robots and killing humans are lines not to be blurred in mainstream 'toons... Creators can do the first, but not the second... You never saw Cobras strafing Ace's chute after he ejected, watching him tumble to the earth at terminal velocity. And really, you wouldn't want to...

I would have answered this CC hands down in the '80s though. Even now, a close second...