Dark Samus's motive is revenge against Phaaze. And she gets that revenge by taking over Phaaze at the end of Prime 3. Dark Samus had the universe in her palm.

Not sure.

I read a theory that said Dark Samus's motive was to protect her species (the metroids) from Samus (with her intelligence she forsaw the demise of her species at the hands of Samus). She wanted to create an environment where her species thrived, which was Phazon. She wanted to create more highly evolved metroids, so she wouldn't be alone anymore (the only intelligent metroid).

Definitely Ridley, followed by Dark Samus!

I just realized I forgot the Ing. And MP2 is my favorite Metroid. ;(

Can't forget the Ing!

Sadly, they're only in one Metroid game :(

Feh. Samus undoes in one afternoon what the Ing spent decades accomplishing. They're so minor a threat they're not even the final boss in their own game.

lol Sakamoto.

I voted for the Federation. They're responsible for half the stuff that goes bad in the series. Surprised I'm the only one.

That's because the Ing only got their hands on the Energy Transfer module late in the war. They can take over technology but not make it themselves. Without the Energy Transfer module Samus would've been possesed by the Ing.

The Sanctuary Fortress only fell shortly before Samus arrived on Aether for example.

Kriad used to be my favorite mainly because he was SO massive in Super Metriod and green. But I've eventually warmed up Ridley, ironically because according to Other M his growth is pretty much that of a homicidal Pokemon in changing forms and I love that!

Mother Brain is so ridiculous is has to be her (/him/it).

Especially the boss fight in Super Metroid, which is still one of the most unique (the good kind of unique) and memorable boss fights.