9) Wave Beam
First Appearance: Metroid (1986)
Despite the large number of powerups Samus gains over the course of the franchise, her main weapon has always been her Arm Cannon. And like most parts of her Power Suit, she's able to find upgrades to her basic beam attack, including the Wave function.
In Other M the Wave Beam is limited only to transparent barriers such as glass, but it's still a cool weapon to use. In fact, the first time you use it is pretty neat; you are running down a hallway when suddenly you are enclosed in a glass case with Space Pirates on all sides. Your CO gives you the go ahead to activate your Wave function so you are able to destroy all of your enemies from the safety of their own trap.
The Wave Beam tends to be found around the middle of the games it's featured in, but Fusion flips the script a bit: it is the second to last beam you acquire (technically the last, as you can only use the final beam in the last few minutes of the games), when you use it to access a secret Metroid cloning laboratory.
The Prime games remove the wall penetration effect altogether, instead having an elemental theme to their beams, with Wave representing electricity. You are able to home in enemies, power up deactivated electronics, and overload enemy turrets.
Overall the Wave Beam is a fun tool to use in all of its appearances, and is definitely one of my favorites to use in any game.
9) Morph Ball
I love the Morph Ball in Metroid games. It gives you an infinite amount of new options. You can explore new locations. Or give yourself a new way to dodge enemy attacks. I also just love the ways the entire Metroid series has evolved this item and done great things with the ability to turn into a ball.