Those Metroid enemies that are unbeatable until you get the right weapon for them. I'd say "SCREW THEM" but when you get that it is so satisfying beating them.
But there's one specific Metroid enemy that deserves special mention.
SA-X
If you're familiar with Metroid games, you should know by the end of them you're normally this overpowered monster with all the equipment you can have and there's nothing that can stop you. In Metroid Fusion, however, throughout the whole game
you, stripped down of your powers, are being hunted by a full power Samus Aran. You are the hunted.
What makes the SA-X so effective is the fact it is absolutely unbeatable. It is scary not because it's creepy, but because it's coming to kill you. And you're too weak against it. In the game, you'll see signs and evidence in some areas of it being there, and throughout the game you have to avoid it seeing you because otherwise it's instant death. No ifs, or buts. OK there are sequences where you DO have to confront it to progress, and that's the only time in gaming did I ever actually scream. Taking the thing down (despite how hard it was) was so incredibly satisfying.
To quote Samus herself:
"The SA-X. An enemy with my strongest abilities. But does it have reason? Probably not. It's just a killing machine. In my current state, I can't face it head-on. The SA-X is me, only heartless. I must stop it before it learns its potential and threatens the universe..."