Every time you drop a Power Bomb, it feels like you are unleashing a bomb of awesome. By then you're so powerful you don't really need them, but that's why it's so fun.
Swimming in lava with the Gravity Suit, compounded if you make it back to Norfair, letting you dance in the now harmless red Kool-Aid land.
Defeating the Chozo Mural and getting your fully upgraded suit back. Complete with the game playing the classic Metroid fanfare for the first time with a choir. Then the Brinstar theme resumes and it's time to slay some Space Pirates!