First, let's look at a number of possible contenders and how they are eliminated: Bruticus defeats Devastator (as seen in G1 episode "Starscream's Brigade") Menasor defeats Bruticus (as also seen in G1 episode "Starscream's Brigade") Ultra Magnus defeats Menasor (Dreamwave comics continuity) Ultra Magnus is more durable than Optimus Prime. (Magnus survives being torn apart, while Prime perishes from much less in TF:TM.) Optimus Prime defeats Megatron (from TF:TM) Sixshot massacres Ultra Magnus (from TF:Headmasters.) Sixshot permits the Terrorcons to live when approached by the Deathbringers to kill them as an initiation. (from Marvel UK G1 comic) Hence, Sixshot is the winner, and the most lethal of the Decepticons in G1.