Metroid II is a very overlooked game. Most people think Super Metroid was the 2nd game in the series.
The game went deeper into the actual metroids. You got sent to their home planet and saw how the metroids evolved. They get meaner and more nasty as the game goes along.
The Queen Metroid is also almost impossible to kill. Wish the game had a remake since the use of a map and some other features could have helped. Heard there was a cancelled DX version with color.