Depends on how you qualify "most powerful". Are you putting any sort of limitations on this? There's going to be a lot of subjectivity involved. Off the top of my head? Here's a short list of 5 Autobots: 1) Primus 2) God Ginrai 3) Big Powered 4) Brave Maximus 5) Big Convoy Decepticons: 1) Unicron 2) Matrix-powered Thunderwing 3) Underbase-powered Starscream / Warworld Empowered Starscream 4) Deceptigod 5) UK comics Galvatron
Unicron. Seriously, he makes mini-hims out of rocks (which aren't technological) and can cross continuities, rendering him immortal. Then there's Starscream's ghost and Injector. Because thy are some hella handsome fellows.
Ignoring Unicron and Primus? The Thirteen have several star-busters among them. Omega Doom was a star killer. God Ginrai and Grand Maximus destroyed a black hole, which Black Zarak created. Cartoon Galvatron was a planet buster. Master Megatron/Galvatron and Super Starscream were planet busters. Superlink Galvatron in his base form was a planet buster. Optimus Supreme whooped Unicron's ass. Zel Quartz Shockwave destroyed Cybertron. It all depends on what you mean by power. Lifting strength? Endurance? Destructive power? Metroplex can shoot antimatter and lift mountains, but cartoon Megatron survived a planet moving explosion. Powerful how?
I'm going to ignore primus, unicron and the 13 as non are autobots or decepticons and for all the know as they haven't really made any proper fiction appearances the 13 could have been wimps Autobots Optimus Prime Rodimus Prime Ultra Magnus Omega Supreme Fortress Mximus Metroplex Grimlock Sky Lynx Starsaber Dai Atlas Decepticons Megatron Galvertron (uk comic version) overlord (idw version) tarn thunderwing (matrix bearing us comic version) Sixshot Trypticon Shockwave Starscream (underbase) Blackzarak these thread will probably get some buzsaw memes in here too
Ha, knew that was going to come up. I say Bumblebee's peg-warming cancels out his selling power though. His real power is the ability to keep showing up the shelves no matter what incarnation of Transformers it is...