Hmm. most powerful, it depends on what you consider power. Is it the realms they own or do they gain power from worshipers, or is how many people are effected by their realms. Within each one is different. For me the first category would have Meridia as one of the most powerful along with Dagon Meridia is the patron of the energies of all living things, lets face it that's pretty powerful. I mean are Daedric Princes alive what about the Aedra. Dagon has Destruction and Energy to name a few, those are very powerful hell maybe more so then Meridia.
For my second idea it would be Azura, Boethiah and Mephala. They are all worshiped heavily by the Dark Elves, almost all of them in fact, and although we know that the Forsworn worship Daedra its not a whole race nor do we know which ones.
And for my final I would say Azura, Hermaeus Mora, Dagon and Sheogorath would be the most powerful. Dusk and Dawn effect everything, not just living things like Sheogorath and Herma Mora but everything. Herma and Sheogorath are both very powerful, every living thing has madness in them to some degree, but Dagon and Azura control larger domains.
Sorry for the long debate i guess but I like discussing these sort of things where I'm given a broad subject with no boundaries, please forgive me if something is wrong from a Lore stand point or if there is a set reason they gain power, I'm no lore Pro.