Each Character has a final smash and each is different.
Super Galaxy Finish: Mario turns into Flying Mario and jumps into the air and starts to fly around the area. A little slower than Super Soinc. After a while, Marios eyes will burn red and he shoots a huge fireball and throws it on the ground.
Boo Attack: Luigi will lead an army of Boos. The storm of Boos can freeze you, put a flower on your head and even burn you! The attack ends with King Boo, who causes all three at once.
Peach Tail: Peach will turn into her Tanooki self; and start swinging her tail from left to right. Then, she flies into the air and ends the attack by crashing into the ground.
Airship Airbound: Bowser will lead his airships from Super Mario Bros 3. The airships will shoot out Hammer Bros' hammers, Bullet Bills, etc. Pretty much everything Bowser has.
Koopa Car: Bowser Jr will hop into his Koopa Clown Car and start breathing fire and tossing Bob-ombs. He ends the attack with a spike ball, which explodes on everyone.
Boomerang Frenzy: Hammer Bro turns into Boomerang Bro, and tosses boomerangs at his foes. He'll also hop around sometimes and the boomerangs have a chance of being super big.