Ganon's up tilt can be used to activate the exploding field on Green Greens. If you have amazing timing, you could also use it to kill and Ike who is using Quick Draw the same way King Dedede does with his Inhale.
Peach Bomber is obviously not useless because it's her best damaging attack she can do while holding a bomb without throwing it.
Things like Mario's down tilt are useful for obscure follow ups as people have pointed out.
Jigglypuff's Sing can be ledge canceled, and if you predict an empty short hop that will land within Sing range, it's a good option.
Mario's F.L.U.D.D. and Squirtle's Water Gun are great for gimping Ness and Lucas if they try to recover from certain angles.
R.O.B. can use his forward special to really own Pikachu when Pikachu is using Thunder.
Stuff like Warlock Punch is obviously useful against a broken shield. That also covers Pk Flash (not as strong as Ness's other stuff, but there's no real loss in diminishing Pk Flash).
Stuff like Zero Suit Samus's up throw might be able to kill in some situations. What if you grab them in the middle of Luigi's Mansion where they could tech both the forward and the back throws but not the up but they are at a really high percentage? Any throw that is the longest or shortest animation throw a character has is inherently useful for either forcing you both to be hit by a hazard or getting them away fast so you personally can avoid a hazard, and any throw that is the strongest knockback you have in any of the four cardinal directions will be your best bet for killing in some circumstances; keep that in mind before naming seemingly bad throws.
King Dedede's Jet Hammer is pretty close to useless since his forward smash is better against broken shields and is fine to diminish. He could hurt himself by overcharging to avoid being chainthrown by Falco I suppose, and I seem to recall hearing that it was really good for hitting opponents doing something obscure that I can't remember.
Pk Freeze is pretty terrible, but if you want to hit a particular spot over and over again with a projectile it can be more useful than Pk Thunder. I saw it work out well in teams once when I had a full bucket and we were both hanging out on the side of the shine gate on Delfino Plaza; it really damaged the enemy's ability to not approach us.
My vote for worst move in the game goes to Mr. Game & Watch's Judgment Hammer, but if you are playing someone WAY better than you that you have no hope at all of ever beating, praying for lucky #9 might be your best bet. In general I'd prefer to try to use good moves to come back if I'm against an opponent that I think I have any hope of ever beating no matter how much I'm down (since Judgment Hammer is a pretty good way to lose), but there is some argument on last stock 0% enemy versus you at 190% or something for trying for Judgment Hammer.
Some moves are really terrible, but it's not fair to say that any move is useless in 100% of possible situations. Even the worst of moves have the best of moments.