The Megatank is the most powerful form of the Tank in Advance Wars. It has three main cannons on the top and two Machine Guns on the bottom. Although it is very powerful, Megatanks cost a lot of money at 28000G (the equivalent of deploying a Battleship), and don't have a very high movement range, so mass producing Megatanks is generally not recommended.
It made its first appearance in Advance Wars: Dual Strike as a Green Earth unit that is very durable, this is enforced by the fact that Javier and Jess are the COs controlling the Megatank. Clearing that mission unlocks the Megatank.
The Megatank is the strongest Ground Unit in the game, as its primary weapon is capable of destroying a Medium Tank unit in one attack at full health. Plus, if you need to park a Unit on a Bridge or HQ, the Megatank would be a prime choice because of how difficult it is to destroy one--it takes multiple attacks from multiple units to destroy one.
A Megatank will draw a lot of enemy fire from the AI in Advance Wars: Dual Strike because the Megatank only has three rounds for its Primary Weapon. The best way to counter that is to have an APC nearby.