General

Cupid
Ninja
And anything that can fly, really.

SuperStar had the best movesets

Umbrella
Yo-Yo
Mirror

Sword
Fighter
Suplex

And im talking about Superstar ultra its the only one i have :[

Sword
Wheel
PEWPEWPEWPEWPEWPEW oh i mean Lazer.

Are we looking specifically at Kirby Superstar/Ultra Deluxe? If so, my favourite powers are:
-Ninja <- can destroy the Helper to Hero arena mode in record time with this one.
-Yo-yo <- favourite if playing as Kirby
-Fighter
[b]NOTE:[/b] If I'm playing as a helper instead of Kirby, Copy is my all-time favourite.

However, I do have favourites from other Kirby games...

Kirby's Adventure / Nightmare in Dreamland:
-Sword <- favourite in this game
-Fireball
-Back Drop

Kirby 64 - The Crystal Shards:
-Dual-bladed energy sword (electric+cutter) <- absolute favourite in this game
-Explosive Shuriken (bomb+cutter)
-Drill (needle+bomb?)

Kirby's Squeak Squad:
-Any version of Sword (normal, lightning, fire or ice) <- I am absolutely DEADLY with this. I've combo'd an enemy across the field with sword!
-Cutter
-Ninja

Kirby and the Amazing Mirror:
-Smash <- favourite in this game
-Sword (sense a pattern here?)

I can't remember which else I liked in this one...

Naturally, I'm not going to mention the special abilities like Star Rod, Triple Rod or Master Sword as they're the ultimate weapons in each game.

Plasma
Sword
and Yo-yo Kirby.

These are from Kirby Super Star (SNES) though.

Umbrella (So useless yet entertaining)
UFO
Spark

UFO
Smash
Fire

That was fast O.o

Regular
Sword
Bow/Arrow