9) Omega Pirate
Featured In: Metroid Prime (2002)
One of the hardest areas of Metroid Prime is the Phazon Mines, where the Space Pirate forces are at their strongest. As you decimate the troopers, you will periodically stumble upon their Elite specimens. If you've read all the logs up until this point, you learn that these Elite Troopers are infused with Phazon (the mutagenic substance you've likely hurt yourself with by that point) to the point where draw vast power from it, like Bane from Batman.
As I entered into one of the final rooms of the lowest levels of the Mines, I met the thing.
This fight is fairly straightforward, as you've fought several Elite Pirates by this point. What makes this one different is that he's able to render himself invisible and call upon other Pirate troops for assistance. He usually ends up killing them all trying to attack you anyway so it's not too big of a hassle on normal difficulties.
Once you've knocked his health down all the way, he collapses on top of you and melts into a pool of Phazon. For whatever reason this doesn't corrupt you, but instead gives you the super awesome Phazon Suit upgrade!
While this fight isn't the best, it's the buildup to his appearance that makes him so memorable. Plus after slaughtering Pirate after Pirate, it felt good to take down a big one.
9) Security Robot B.O.X. (Metroid Fusion)
I love this fight. You get to shoot at this giant robot from above while hanging by your fingers from a grab rail. And if you fall while fighting him he tramples o er you. To my memory there is no other fight like this in a Metroid game! Even though this boss isn't that hard it's still a fun fight that has stuck with me.