M2's down+b is not useless as such. It has surprising range and leads into a free shadowball or grab, and even though it's tricky to time, it is hardly useless.
And yes, I know what I'm talking about, because I have seen a VERY good Mewtwo. Just ask Taj, and if you don't know who he is, ask PC Chris or Caveman.
His forward+b also yanks people under the stage in Pokemon Stadium and Battlefield, generally KO'ing them.
DK's down+b has less lag than his forward+b if you don't keep holding b... just tap it, you'll slap the ground once and then be free to move. His forward+b is very easily meteor cancelled. I do not like it at all.
Luigi's dash attack is pretty bad... even if you hit with it most opponents are easily able to counter it.
Ganon's u-tilt definitely will KO an opponent sooner than his warlock punch will. If Jigglypuff misses a rest or an opponent's shield breaks, you will be happy to have this move.
IC's d-air and u-air... bad?... What? U-air has reach and priority and will go through many moves and is handy for quick juggles. D-air = chain grabs, and when fast falled and l-cancelled can give you a grab if you time it well.
Kirby's n-air is not very good. I don't know when I would prefer to use this over b-air, f-air, or u-air. Then again, I'm not a Kirby pro. I guess you could use it for a silly psuedo-spike on a fast faller, like Falco or Fox, right as their up+b starts. It will send them out and down and give them a mean recovery time to do another one. And since it doesn't pop them up any, there's no real risk you will help them to the stage inadvertantly. That's all speculation though.
Yoshi's forward+b can, in rather rare circumstances, provide a quick escape to the other end of a level if you're above an opponent and don't have your second jump. Not much for actual damage use, but... oh well.
Umm... that's all I got for now.