Let me say right off the bat that you shouldn't judge a superhero by his or her powers. Batman has no powers whatsoever, yet he is regarded as one the greatest superheroes of all time. That being said, if I had to pick a superhero with the lamest, worst power it would have to be Zan of the Wonder Twins.
Now there may be some comic book aficionados out there that are aware of someone with worse powers, but turning yourself into a bucket of water is pretty damn awful. The question I always had was where'd that bucket come from?
A while back they revamped Aquaman to look like more of a bad ass, I guess. I think he even had a hook for a hand. They tried the same sort of thing with the Wonder Twins, allowing Zan to transform into a frost giant - bye-bye bucket of water, hello demonic whirlpool.
In contrast, I've always thought Jayna's powers were pretty cool. I guess everything from a bacterium to a blue whale is fair game. With the revamped version, she could even take the form of mythical creatures like the hydra. This sort of reminds me of another superhero from the small screen....
You guessed it, Manimal! But if you're going to bring up TV super powers, there's simply no comparison to Samantha Stephens and Jeannie. Their powers are godlike - they can teleport, alter time, create matter from nothing... basically, they could own the universe if they were the type. Yet, Darrin won't even let Sam use her powers to wash the dishes. Sam could take on Martian Manhunter and Firestorm at the same time and not break a sweat... but I digress. This is the sort of thing that you come up with when you have too much time on your hands. But I'd still love to see the unstoppable force of a Manhunter-Stephens team up.