Metroid: 25 Greatest Boss Fights RANKED
4. SA-X - Metroid Fusion
Most of Metroid Fusion's bosses may not be particularly iconic but the antagonist is the scariest villain the franchise. Throughout the game, you are constantly hunted by a clone of Samus Aran that's fused with the X parasite. This entity, SA-X, has emulated Samus' appearance, as well as her weapons, power-ups, upgrades, and armor.
Because SA-X is basically Samus with max stats, she is literally unbeatable until you've obtained every power-up in the game. Facing her before you are ready is suicide so you have no choice but to run or hide when she draws near. Although the final boss battle with her is pretty standard, the terror that the player experiences throughout the game makes SA-X one of the best bosses ever.
SA-X was so popular, the concept of being hunted by an indestructible enemy was reworked into the EMMI robots in the sequel, Metroid Dread.