Off the top my head I Galvatron who's literally Megatron anyway. least allot of of the time anyway. Shockwave. At least two continuities were he either tries to takeover the Decepticons or tries to conquer cybertron itself. Tends to use Triplechangers sixchangers and duocons as his subordinates since in some univereses their results of his experiments. Straxus. ruler of polyhex. A relatively small minded tyrant content with staying within his current;y claimed territories. Is a very powerful and brutal fighter to the point even Megatron knows it's wise to give him his space and not go poking a sleeping predacon. Overlord. I know he's a decepticon leader in one of the japanese series and in power of the primes Megatron is one of his main rivals. He certainly seemed capable of making a competent power play if he wanted. Scorponok. In some continuities he's leaderof his own decepticon forces main the head target and power masters. But being controlled by the more intelligent and cunning Zarak in the animated series Zarak could easily make a successful powerplay against the deranged Galvatron(He nearly blasted his own men dead when he learnt and was disgusted they'd allowed organic creatures to upgrade them even though the process greatly improved their combat abilities) Are their many others?