Which Decepticon leader has a fusion cannon on his arm?

Megatron is the Decepticon leader equipped with an arm-mounted fusion cannon.

The fusion cannon is one of Megatron’s defining weapons, especially in the original Generation 1 design. Mounted on his right arm, it fires powerful energy blasts and visually reinforces his role as the Decepticons’ heavily armed commander. Depending on the continuity, Megatron can also transform into a handgun, tank, jet, or other weapon platform, but the arm cannon remains a recurring part of his character design.

The alternatives have different identities. Starscream is Megatron’s ambitious second-in-command and commonly transforms into a jet. Shockwave is a logic-driven Decepticon scientist and commander whose weaponry varies by continuity. Galvatron is often Megatron’s reconstructed successor after Megatron is defeated and reformatted by Unicron in The Transformers: The Movie, although some continuities treat the two as separate characters.

Modern films and series reinterpret the cannon, sometimes making it handheld or integrating it into a different arm design. Even so, “Decepticon leader with a fusion cannon on his arm” is a canonical description of Megatron.