The whole question of relative power is confusing to me. I guess I just never thought of it like that. Its a perfectly valid question, but I never imagined the gods as people with power x, y and z. They are the manifestation of their dominions, as much a prisioner to the aspect they personify as they are its controller.
The gods that oppose each other don't do so out of rivalry's and the typical biases of polytheistic mythology's. They oppose each other because their aspects are opposed. Bhall is the goddess of fire, but her real aspect is sudden dynamic change. Because of that she opposed Mulcarn the god of winter whose dominion represented stasis. Bhall opposed Sucellus because the nature gods aspect is growth, slow change. Where Bhall wanted to raze and rebuild, Sucellus wanted to nurture and strengthen. Mulcarn also opposed Suceulls (though not to the degree of Bhall) because even the slow change of Sucellus was in opposition to his permanence.
And so on and so on. I guess I saw the god as primoral forces, they definitly have a mentality that guides them. But asking which is more powerful is like asking which is more powerful, the slow growth of a great forest or the forest fire that sweeps through it. In truth any aspect taken to its extreme (which every god would do) isn't healthy, which is why we have so many iterations of the story of fanatism on any front leading to ruin (or even ending up twisting around to opposite end of the spectrum).
So I dont know about which is the most powerful. I always kind of thought it would be Agares. He is brillant and terrible. In his own mind the whole experiment of creation is flawed and he wants to wipe it out. He doesn't want to kill the other angels (not that he is powerful enough to simply do so) he really wants to convert them to his service. Then rebuild the bridge to heaven and attack the One directly. He views the One as the keeper of power and their jailkeeper in the prison of creation, and Agares is ready to break free rather than fight for the crumbs. So he spends his centuries alernativly mopping in his deepest hell or trying to slowly whittle away at those that stand against him, trying to get them to Fall as Bhall has.
But yes, Arawn would have been powerful to. He didn't get involved in the gods war (Danalin and Sucellus didn't either) and has forever guarded the borders to the source of life and the underworld. He may be powerful, but he is the least likely to influence anything happening on creation.