First off Mewtwo's UpB is the reason I started playing him. Its probably about the third best UpB in the game. Also, guys you need to stop saying that a certain move is useful because "every move is useful in a certain situation". So what, what makes a move good is versatility for the most part. A move that can score a big hit once every 20 matches is a bad move, no matter how good it is in that situation.
That being said, it seems the general concensus here is either Sheiks downB, or Ganon's Utilt. I have to agree, probably the two worst, picking just one would be kinda hard but I guess I would go with Sheik's.
Also here are some moves people should stop listing.
Yoshi's upB. Seriously guys, I haven't ever even played Yoshi and I know the uses of ledge canceled eggs.
Bowser's downB. Bowser has the best near-ledge game there is, period. This move ledge canceled has tons of options, I use it fairly often.
G&W B. Its a horrible projectile sure, but it does some other things. Most obvious is the pan spike that is pretty improbable to try for, but it also stops all horizontal movement in the air similar to the shine. That can be useful if your clever with it.