Three options comes to mind for each main series.
1)
Full Power G1 Galvatron Often noted (particularly by Simon Furman) as the Ultimate Decepticon, a virtually invincible powerhouse and before the Insanity often kicks in, he is without equal. Even in Sunbow, during The Rebirth, Ultra Magnus as well as a dozen Autobots were shooting him point blank. Which by Magnus' own words was only merely "weakening" him. Plot convenience was his only weakness
2.
BW Rampage - with complete sparkRampage was a virtual force of nature within BW and that was with half an "immortal" spark. Again he was only beaten at full strength by terrain/plot convenience. Given his nature, raw power and the overwhelming arsenal he commanded I think he was the most powerful in Beast Wars.
3.
IDW SixshotThe -Ations fight against Prime was BS. There is a reason the Six Changers are used so sparingly and with his noted feats as a Phase Sixer within IDWverse, Prime was going to lose an on-panel fight against such an opponent. I shouldn't really have to sell the power of a Six Changer here, especially when he was taken so seriously during the -Ations, he has been largely absent ever since. A little hard for an unstoppable Decepticon Army of One to fit in with the tone of either the humanising MTMTE or the political RiD.