I recently played a great retro-themed platformer, Midnight Manor that offered the player various power-ups during the game and after receiving the double-jump power-up, I was reminded how much I love that power-up and how it is an excellent way to push the game forward, by unlocking previously unexplored areas in a very natural way.
It made me think of other power-ups that people may love in one game or various games as the double jump ability is certainly not tied to one item only.
This might not be quite what you’re looking for, but I always love it when you can become Super Sonic in those classic Sonic games. (And even Hyper Sonic, in the case of Sonic 3 and Knuckles.) This felt mind-blowing when it first happened in Sonic 2. Upon activating this power, you’re suddenly going faster (in a game that’s already fast), you’re jumping higher, you’re invincible, just plowing straight through all the enemies, and that energetic catchy tune is playing all the while… You are an unstoppable force! It’s definitely a reward that feels great to receive and utilize, after completing the optional but lofty task of collecting all the Chaos Emeralds.
What keeps Super Sonic from being too OP though is the fact it’s not permanent (it only lasts as long as you have rings, which steadily deplete), and if you’re not careful you can still die easily enough getting crushed by traps or falling into pits.
Pits that include… the infamous deep spike-floored one in Mystic Cave Zone. God help you if you fall in and you’ve got over a hundred rings.
I would classify Hyper and Super Sonic as fitting the criteria as I like to think of a power-up as being anything that alters your characters abilities beyond what you normally start with, whether it be a Fire Flower for Mario, Super Sonic transformation from gathering the Chaos Emeralds for Sonic and team, to others.