Queen Metroid

The Queen Metroid is the largest and most powerful Metroid featured in Metroid II: Return of Samus. It is best compared in structure to a crocodile; the creature has a quadrupedal posture with a large broad jawline containing three rows of teeth. The creature also has eight eyes in total, (two groups of four) and an extendable neck used for attacking with its impressive mandibles. Visually, the creature's features don't differ from the Omega Metroid's in any major way; only a few aspects change, such as the mouth and spinal plating.

The Queen Metroid was the one responsible for reproducing the entire species, much like a "Queen" of an insect colony. It is unknown if the Queen needed a partner to mate with before being able to lay eggs, or if it was intentionally designed (by the Chozo) to not require one. This creature thrived in the deepest confines of the entire cavern complex of SR388, in her own lair. This was the final Metroid that Samus destroyed on her mission to the planet, but she took the Metroid Hatchling with her as she left.

Battle Tactics
The Queen Metroid, as stated, is the queen of the species and her destruction halts the reproduction of the Metroids. Destroying the Queen requires at least 180 Missiles in stock. Upon Samus's arrival, the Queen will immediately attack with her huge mandibles, which can be dodged simply by a well-timed jump. When the Queen opens its mouth to attack, it can be briefly stunned in place with a Missile, leaving it open for more attacks. The Queen will also periodically launch globs of fluid from its mouth that track Samus, though they can be avoided with a Screw Attack. Should Samus be unable to win the fight, there is a small passage below the Queen that leads out of the nest. It can be used as an emergency escape, though the Queen will have healed all damage when Samus returns.

There is also a more effective way to destroy the Queen Metroid. After stunning the Queen with a Missile, Samus can instead go into Morph Ball form and enter its mouth, then roll into its stomach and plant Bombs (Samus will be damaged constantly while inside the Queen, so she can't delay). The Bombs will severely damage the Queen and Samus will be spat back out. It only takes about five attacks of this method to destroy the Queen. Once the queen is destroyed, she falls to the ground, her corpse disintegrating while Samus' Energy is somehow fully restored. The Queen dead, Samus has access to the last Metroid Egg and her Starship.

Trivia

 * The Queen Metroid is one of the four known types of Metroids capable of spawning other Metroids, the other three types being Fission Metroids, Metroid Hatchers, and the Metroid Prime. The Queen Metroid is also the only non-Phazon Metroid capable of reproducing other Metroids.
 * As the mother of all Metroids on SR388, their homeworld, it is quite possible she was the first Metroid ever created by the Chozo.
 * It is possible that a Queen Metroid goes along its own evolutionary path from Larva to Queen status. This is supported by the fact that Adam Malkovich makes no mention of the Queen when talking about the secret breeding programs going on at BSl and speeding up the growth cycles of Metroids. He only talks about advancing a Larva to an Omega in a matter of days. It is also a possibilty that the Federation had no intention of producing a Queen, since it would have started producing its own Metroids other than the ones created through cloning (which might have complicated matters).
 * As the Metroid series is known to have taken inspiration from the Alien film series, it is entirely possible that the Queen Metroid was inspired by the Queen Alien from those films. Both Aliens and Metroid II: Return of Samus depict a final battle between the queens and the female protagonist (Ellen Ripley in Aliens and Samus in Return of Samus).
 * It is possible that the Queen Metroid knew of Samus' presence on SR388, and the fact that she was killing the Metroids. If true, the Queen may have been protecting the last Metroid Egg from Samus in case she intended to kill it.
 * During the fight with the queen, there is a strange meter on the wall that increases when the Queen's head nears Samus, and decreases when her head retracts.