I imagine it’s Q, but lets do a poll anyway.
Nagilum
“We’re omnipotent.”
That pretty much sums it up for me.
Although that guy who wiped an entire interstellar civilization out of existence with a thought just because they killed his wife should be up there, too.
Second link is the same as the first. Here is the correct link.
The douwd.
Q actually brought Picard back to life (and then made him relive it as a wuss.) I don’t remember any other being pulling that off.
Based on reading the episode summaries, I’d say the scriptwriters.
Thanks for that. 
The Sisko
He punched out Q and got him to leave forever.
Guinan said the Q arnt as powerful as they would like you to think. The series bears that out.
As pointed out earlier Kevin Uxbridge destroyed all of that alien race everywhere. The Q had to use some kind of convulated anti-time plot maguffin to wipe out humanity.
Then there are the wormhole aliens.
When did she say that about the Q? I thought the Q had total control over our dimension.
I got the picture that the Q could be omnipotent, but were restricted by the Q Continuum (so if Q wanted to do something sufficiently crazy, the Continuum would stop him).
I don’t think there’s any such restriction on the Douwd. If Q wanted to come after Kevin Uxbridge, I would bet on Kevin Uxbridge to successfully evade him. If Q killed Kevin Uxbridge’s wife, I would bet on Uxbridge killing him (or getting close and then the Continuum saving Q).
Research shows that it was probably Quinn (The excellent Voyager -IMO- ep “Death Wish”) who said that
I don’t think he did that at all. My understanding was that Picard created the time anomaly. Q then stepped in to help Picard solve the problem.
The Q manipulated Picard into doing that…Q did step in and help cause he likes Picard…
It’s very complicated ok!!!
Didn’t Q tell Gordie to alter the gravitational constant of the universe like it was no big deal? That’s some serious shit.
Q can’t even eat one hot fudge sundae
The Nagilum was apparently limited to a hole in space. And the doud, while he could kill entire species, seemed to lack the ability to just move his planet somewhere else, or hide it from sensors, or make the crew forget about it–stuff a Q has been shown the capability of doing.
You shouldn’t listen to her. She’s an imp.
Oh, just our dimension? How limiting.
Kevin the Douwd specifically said that he did not have the power to raise the dead back to life – that’s why he was so heartbroken and enraged by the death of his wife. On the other hand, we know that Q can - as well as create new beings at the snap of his fingers, and casually alter the physical properties of the universe. It’s really no contest.
I really messed this poll up. I think the better question to ask is: "Who do you think is the SECOND most powerful entity on ST:NG?
Oh well… :o