User blog comment:AustinDR/The Worst PE Villains/@comment-2175012-20171023023858/@comment-27022982-20171026225658

I consider Unicron; he is ultimately the most evil villain in the Transformers series so far, as he wants to devour the multiverse. Although Megatron sometimes joins forces with Unicron, the latter is using him as a pawn. He enjoys devouring planets and everything that inhabits them. Unicron also has a twin brother named Primus who is trying to stop him, and he will do anything to destroy Primus.

That's why I think Unicron would make a good candidate.