Favorite without a doubt is fighter. I'd do fighter-only runs whenever possible. It's very versatile, from doing rapid-fire hadoukens via quarter circle inputs, to grappling enemies/stars, and it you feel fast af. Its such a satisfying ability to use. Though I liked the elemental swords you could make in squeak squad. Very fun stuff. Oh and my favorite from Kirby 64. Fridge Kirby. Combine Ice and Plasma and it allows you can turn into a fridge that shoots out food out your enemies. Food that makes your enemies explode violently. Oh yeah and it heals you if any food is left over
Call me a sucker for the classics, but my favorite copy abilities have either been Wing or Sword. Wing offers really incredible aerial movement wherever it's available, alongside some super fun moves that combo into each other very smoothly. And Sword is just always a fun ability which always feels really good to take down enemies with. (Alternatively, just let me play Meta Knight, and I'll probably be happy.)