My favorite ability that Kirby has is the ability to copy others' abilities. (I also like the B+down attack on SSBM where he becomes a heavy block of some sort and squashes all who are unlucky enough to be underneath said block falling from above. Idk if he still has that attack in SSBB.)
Ninja, Wing, Sword, Animal and Yo-yo
Ghost in Squeak Squad was cool.
Otherwise, Rock in Super Star Ultra.
Hammer and Yo-yo Kirby from Kirby Super Star were the most spammable and fun to use.
Cupid/Mirror/Beam/Wing/Sword/Burning/Yo-Yo(FTW!)/Hammer (it's powerful)/Animal (ITS EPIC!!! XD)
But I haven't played in awile lol xD My most favorite if I had to pick one is probably Cupid for his sheer cuteness^^
Spear Ninja Fight, Leaf bomb
Also High Jump, Plasma/Spark, Beam, Cutter, and Ninja.
Ninja, Suplex, Yo-yo and Fighter.
I'm gonna be weird and say that I really like Stone. Sometimes it;s good to be defensive.