My favorite ability is Parasol. It was Sword, but I don't like how sword evolved in Superstar and recent games.
Parasol is simple but has enough deepth at the same time, and it's a special ability because it allows you to attack in several ways and to glide instead of flying, that's useful to change the trajectory of the jumps without slowing down too much.
Also, in Kirby's Dreamland 3, Parasol can be exploited as hell, and in Kirby Triple Deluxe you can kill Flowery Woods by just crouching near him and moving+jumping to avoid the attacks in the second part.
Hammer is a second place, for being what Sword was in the old games.
My favorite ability overall, based on a single specific game, is Sword in Kirby and the Amazing Mirror.
Video made by me with a friend, MANY years ago (the gameplay is by me).
It looks like a Smash Bros match in the first part of the video, lol