Decepticon leader

Among the Decepticons, there is one guiding principle: rule of the strong. Whereas the leader of the Autobots consults his subordinates, the strongest and the most powerful Decepticon leads, and everyone else is expected to follow without question. But strength of body is not enough. One must also have the brains to lead, the intelligence to outwit their rivals and plan their victories. Thus, the Decepticon leader is the strongest and smartest of their ranks.

Guess how many Decepticons think they qualify.

Generation 1 cartoon
At first, the Decepticons were led by an old Decepticon who could not transform. However, when he lost the Second Cybertronian War against the Autobots, he was replaced with Megatron, who led the Decepticons to conquer much of Cybertron. However, when Megatron and Optimus Prime disappeared, the war on Cybertron entered a stalemate. Shockwave had been appointed Guardian of Cybertron in Megatron's absence, and became de facto Decepticon leader. Once Megatron resumed contact with Cybertron, Shockwave resubmitted to his authority.

Air Commander Starscream, long contemptuous of Megatron's leadership, made several attempts to supplant Megatron. They all ended with failure, usually due to his own impatience or the other Decepticons' loyalty to Megatron. However, Starscream eventually decided to get some help. He made plans with Blitzwing and Astrotrain to trap Megatron in a tunnel and freeze him. However, the Triple Changers betrayed Starscream, leaving them both to freeze, and proceeded to wreak havoc. Megatron and Starscream were freed due to an accident on Astrotrain's part, and after defeating Astrotrain, Blitzwing, Devastator, and Starscream, Megatron resumed leadership of the Decepticons.

Shattered Glass
Unlike previous incarnations, the leader of the Decepticons is a heroic resistance leader, intending to free Cybertron from the Autobots and save the galaxy from their evil ways. Megatron stands as their current leader, and it seems that Starscream will be a successor rather than a usurper.

Trivia

 * Unlike the Autobots, whose leader is called a Prime, the Decepticon leader has no "official name".
 * Being Decepticon leader means that Laserbeak has to perch on your shoulder. Seriously.