If an up-grade counts as a power-up, then I'll go with the Gilded Sword from Majora's Mask.
If that just counts as an upgrade, then I easily go with the Fire Flower. I'm too retro. :3

What would you say is your favourite power up?
DotA's regeneration rune. Invisibility is nice too, but more situational, while there's almost always someone who could make use of a regen.

And how does it help your play style?
I'm not good at mana management.

The Red Shells from Mario Kart games are one of my favorite power-ups. It hits whoever is at front of you at a hundred-percent accuracy. This gives you the chance to stall them for a few seconds, allowing you to get ahead. Poké Balls from Super Smash Bros. are another of my favorites. They call random Pokémon out on the field whenever you use them, who help you to take down your foes. I also like Invincibility Stars. They've been featured in a variety Mario games throughout the years, and they granted you the power to defeat any enemies as soon as you got into contact with them. d:

The big mushroom in New Super Mario Bros. I loved breezing through the stages with it! Also, the smash ball in Super Smash Bros. Brawl.