Final boss

From Wikitroid, the Metroid wiki

A final boss is the last boss you battle in a video game. In the Metroid series, all games have a final boss which are more difficult than the previous bosses encountered in the games. When you fight these final bosses, it is a good idea to have all Energy Tanks, Missiles, Super Missiles, and whatever else may help you fight the boss.

List below are the final bosses from the Metroid games.

Metroid	Mother Brain
Metroid: Zero Mission	Ridley Robot
Metroid Prime and Metroid Prime Pinball	Metroid Prime
Metroid Prime Hunters: First Hunt	A green Samus (Holographic projection)
Metroid Prime Hunters	Gorea
Metroid Prime 2: Echoes	Dark Samus
Metroid Prime 3: Corruption	Aurora Unit 313
Metroid II: Return of Samus	Queen Metroid
Super Metroid	Mother Brain
Metroid Fusion	Omega Metroid