Personally I think that his spin attack (up-b) is his worst move. The reason being i almost never use it. Now I use it alittle more just to expand my play and keep my opponent guessing (cant always utilt or nair off a grab). But seriously what is it for? You can hardly edge gaurd with it, the only time I reall find myself using it is when I'm recovering from off the stage.
I think Young Links move useless move is his U-smash. Now I know that it CAN be used, but WHY? It leaves you open to getting owned if you miss. And the u-tilt just always seems to be a better choice. Now I do use this move, mostly by accident however.
I think the move I use the least with YL is probably his dair, I used to spam it all the time. I just dont seem to use it much though now, the lag time kills me, and i usually settle for the nair. I know the dair is a good move but I just dont use it that much, probably not as much as I should.