Thinking about Q, the various entities that Kirk ran into, the future folks from the Temporal Cold War, Species 8472, the guy who got Voyager lost out there, etc. Who are/have been/will be the most powerful beings in the universe (not just galaxy)?
0 (Zero or Nil) ranks pretty high up there... as does (*)... Pretty sure the God of Sha Ka Ree is on the list as well... not as high as Q, 0 or (*) though... M
I think the most powerful would be the Q, the Organians, and the Metrons. Possibly whatever new life form that was born out of the merging of Decker & V'Ger. (*) was literally laughed away. The One was destroyed by Klingon disruptors. (Or at least severely crippled by them) Gorgon took his power from children who are fickle anyway. 0 would have been interesting had he actually appeared as a threat on any of the series. And Gary Mitchell would have been a threat but he was thrawted by his old nemesis: a boulder.
It was quite funny how many superbeings there were on TOS. One of the things I really liked about TNG was that they created a single superbeing and took the time to develop the character over the entire years.
The funny thing about superbeings is that they usually don't have to follow the annoying conventions and limitations of linear time. Which means that even if we catch them at a moment of seeming weakness (usually of their own doing, as the Q were very adept at creating boulders they themselves couldn't lift, and didn't need Kirk to do it for them), they may well have been more powerful earlier on and later on, and this moment of weakness is just them being young and silly. We all know and hate the "small universe syndrome", but it does make in-universe and dramatic sense that every superbeing would indeed be "young Q". That is, if you are a superbeing of any worth, you will continue to evolve, and you will reach perfection, and there is just one perfection, which is unity in Q. Of course, it then follows that Pakleds would be future Q, too... Timo Saloniemi
The novels give the symbol (*) to the beast that fed on negative emotions in TOS "Day of the Dove". Apparently, 0 was an entity invented by Greg Cox for his Q Continuum novels that sort of ties the otherwise forgettable (*), along with dozens of other superbeings, to the Q concept. Timo Saloniemi
0 is similar to Q, but from an alternate universe. As a result, it seems that many of the rules of the Star Trek universe don't apply to it. Asides from the non-corporeal beings, one could make a case for the Borg, at least where Peter David's Before Dishonor is concerned. In that novel, the Female Q materialises on a Borg ship and is eventually told that while she has been speaking, the Borg have been analysing her and chillingly reveal that it's only a matter of time before the Borg conquer the Q. Even in live-action Trek, Q seems to hold a certain caution towards the Borg, especially when he tells Q Junior that "You don't provoke the Borg!"
I am kind of split on the Borg vs Q front. On one side the Borg are able to assimilate non-humanoids/non-beings (I am not sure where I got this. I think it was something Seven said in VOY "by the way" when she was talking about some acquired technology). Additionally, the Q Continuum may be an extra-dimensional realm and the source of a Q's power - as was hinted in VOY. SO the Borg may be able to enter. (something of a cross between fluidic space and Unimatrix Zero). On the other hand "You don't provoke the Borg!" may just mean that they will went their frustration on lesser species. But to the OP question we should make a list: 1. Them 2. P Continuum 3. Q Continuum 4. M Continuum (Manraloth and co?;Melony?) 5. 0 6. Organians 7. The One 8. Metrons 9. Nacene 10. Gorgan 11. (*)