Can't beat Beast Wars Megatron. Delightfully cunning and wicked - I love how he was always operating his own agenda; and often had agendas hidden within agendas that kept others second-guessing his true motives. Also love how he manipulates both allies and enemies into ultimately serving his own ends.
Oh yeah, and he did ultimately win the Beast Wars too - how many other Megatrons can claim to have conclusively won in their respective series?
Btw, who the blazes is "Special Edition" Megatron??? And TF: Prime? I don't think anyone here can really judge TF: Prime Megatron considering the series isn't even out yet. :/