Metroids are incredible enemies, perhaps some of the most frightening foes ever created. They are incredibly fast, tend to ambush Samus from the background, and are persistent at swarming her. Should one latch on, it takes several concussive blasts from your bombs to shake them off, and that usually is only for a few seconds before it recovers and homes in on the player again. And it requires some specific weaponry to kill — the Ice Beam to encase it in ice, and then five missiles to break them apart for good.
Of course, the iconic “jellyfish”-like larva are the most prominent and iconic in the series. These are still my favorite, despite Metroid II’s expansion of the family tree of the species. But the Queen Metroid from that game is a fantastic boss, and Omega Metroids (which appear in II and Fusion) are certainly tough to bring down. And then Metroid Prime‘s titular final boss is also pretty damn epic. And Samus is subsequently part Metroid herself down the line. So with Metroids, there’s lots to love!
In summary, Metroids are legendary foes that deserve the spotlight they have been granted as serving as the title of the storied franchise. They evoke horror, fear and terror whenever they become involved, and that kind of visceral reaction is ideal for an enemy to express.