My favorite Kirby game, as well as my first, was Kirby's Adventure (which was later remade as Nightmare in Dream Land). The thing I liked best about it was the MetaKnight boss battle, which was the most epic boss battle of my childhood. I also liked Super Star Ultra, The Crystal Shards, The Amazing Mirror, and Return to Dream Land.
My favorite abilities are Sword, Fighter, Hi-Jump, Smash (the ability from Amazing Mirror that gives Kirby his special moveset from Smash Bros.), Laser, Fireball (which was renamed "Burning" in Nightmare in Dream Land for some reason. "Fireball" just sounds cooler), Spark/Cutter from Crystal Shards (the dual-bladed lightsaber), and Master (the ability using MetaKnight's sword in Amazing Mirror).