Sailor Cosmos may have had the ability to be the most powerful, but she lost her whole dimension in the Sailor Wars, and it took Eternal Sailor Moon to get her back on her feet. Sailor Saturn can destroy the universe, which isn't something to be taken lightly, but as for her super powerful powers, that's where it ends. Her normal, keep everyone alive, powers aren't anything that can rival Eternal Sailor Moon, who stopped Saturn from destroying the universe, anyway. There really isn't any competition here. Eternal Sailor Moon is even stated as the most powerful Senshi in the manga multiple times, perhaps in the anime, too.

She is very powerful.

I vote for Saturn. In fact, Sailor Cosmos never can exist. No senshi have a most power in universal.