Besides ones that make one invincible, what is the strongest power up?

[Cell]

I say Bunny Mario, those must be some powerful ears to carry a big plumber and an even bigger plumber, any Mario game is welcomed but I'm choosing to narrow it down and not take power ups from other franchises that Mario crossovers with

 

[Movie Bowser]

The Gold Flower.

 

[Hot Cocoa]

I hate to admit it, but the Penguin Suit. It's practically the Ice Flower and the Blue Shell mixed into one.

 

[Uniju]

frog suit. makes water levels bearable. plus, it's a frog

 

[LeftyGreenMario]

Stouter than an iron burrito... Studlier than a snow tire... Faster than a galloping mule!

He is... er... I am...

WAAAAAAAAAAAAAAAARIO-MAN!

He can even stop a full-moving train!!!! :^)

 

[Mags]

Here's a list of a bunch of powerups that come to mind and the series they're found in:
Smash Ball (Smash Bros.)
Penguin Suit (New Super Mario Bros.)
Koopa Shells in general
Bowser Candy (Mario Party 8. Speaking of, have you signed up for the Trick-or-Treat yet?)
Stone Copy Ability (Kirby)
The highest upgrade of the Poltergust 5000 (Luigi's Mansion Dark Moon)
Lucky Bell (Super Mario 3D World)
Blue Shell (Mario Kart)
Pegasus Boots (Legend of Zelda)
Fairies (Legend of Zelda)
Energy Drink (Brawl Stars. Not that anyone cares of course. : (
Red Shell (Mario Kart)
Cranky Kong (Donkey Kong Country. Not technically a powerup but I'm counting him as one.)
Extra Life (Donkey Kong Country)
Squawks (Donkey Kong Country)

There's probably more that I can't think of at the moment. (Also I'm going to post a list of Brawl Stars stuff later here as well if that's all right)

 

[Flowey]

Y'all really sleeping on the tanooki suit? Flight AND the ability to become a statue

 

[Paper Mario]

The Dark Star, most standard Mario power-ups will likely provide one extra hit against the wave of pure darkness and evil that cannot be avoided and will cast the world into darkness, despair and depression. You'd need teamwork not only with another plumber but with your biggest rival to stand a chance against it. Stone will crumble, flight will run out, fire will be snuffed, fursuits will eventually be reduced to dust in the eternal wind of time. But the night is forever. Mario is sleeping on it

 

[zel]

cape mario he can fly without having to mash b all the time !

 

[Teedgee]

i feel like the cloud flower from galaxy 2 is much more broken than ppl give it credit for, since youre also WAAAAAAY floatier with it on top of the cloud platforms, and i love it

Not sure if I'd call it my all time favourite mario power up but it sure is a contender

 

[Ihsan997]

Y'all really sleeping on the tanooki suit? Flight AND the ability to become a statue

You beat me to it!

 

[JBX9001]

Hmmmm... I remember making a video about top ten power-ups, years ago. Not counting any that grant Mario invincibility, the clear choice has to be Cape Mario. While Mario has multiple power-ups that give him the power of flight, this one is easily the most efficient of the bunch. It may not be free flight like the Red Star gives to Mario, but it is way faster, and unlike the Raccoon ability (when not gained through the P-Wing), Mario won't get tired and can maintain flight for an entire level if you maintain the rhythm of how it works. He can also dive bomb enemies while flying, slow his descent (which makes precision jumps easier), and spin in place indefinitely, which can defeat a lot of enemies in one hit. Mario can even move across the ground while spinning; as long as projectiles aren't in the way, you can mow down almost anything in your path! The fact that the Cape ability has been exclusive to Super Mario World for all these years and has not become as much a mainstay as the Fire ability is downright criminal in my eyes! It needs to come back!