....The Red Star in Super Mario Galaxy. Not because it's a bad powerup, but because you never get to actually use it to complete certain missions or whatnot, which makes me kinda disappointed.
The P-Balloon in Super Mario World. Not because of what the powerup does...It's when you get hit while you have it. You suddenly plummet. Too many a time in a SMW romhack have I seen that essentially cause instant death.
Also, I'm not sure if items in Paper Mario games count, but...the Dry Shroom heals only 1 HP, and since it's a Shroom, we can assume it was once a useful powerup.
As for other powerups...
I liked the idea of the Frog Suit, and it's still handy for big jumps, but the bouncing takes getting used to.
Propeller Suit gets that drill stomp which, IMO, was quite fun to use.
Kuribo's Shoe protects you from munchers. That alone, I'd say, makes it useful.
Rock Mushroom wasn't that hard to control (maybe I was used to rolling ball controls?)
And Spring Mushroom was at least an interesting idea for challenge.
