I'm quite surprised that so many people here didn't play this gem until much later but it just goes to show how old I am I guess, lol.
My favorite parts were the Crocomire battle and the outstanding final battle. Oh, and I love how they paid homage to the original Metroid by having another "fake" Kraid before you fight the real one.
Amazing gameplay, atmosphere, and level design, Super Metroid is easily one of my favorite games ever made.