Ice Climbers look like they have the single most effective FS in the game. Of course, if the level your on allows you to jump off the 'berg then it may not be so effective.
Pit's was very good, as was Snakes and Links. Many seems to be effective if they land it.
Ness/Lucas did very little but seems near impossible to avoid.
Overall, they don't do as much damage as I thought, and aren't the degree of threat that they seemed to be made out to be. The hammer is still much more powerful, esp. the golden hammer.
Still, big plus for whomever lands it, and a good chance you'll knock out weakened foes.