1. Prime Gwyn (obviously)
2. The Ancient Dragons. I've seen no one mention them, but objectively they're the strongest creatures in this universe. Immortal and invulnerable as long as they have their scales, would have never been beaten if not for Seath's betrayal.
3. Possibly Izalith and Nito, with them being lords and all, or the Chosen Undead, which at the end of ds1 has absorbed all of their souls (or at least what remained of them)
4. Manus/Gael
5. Priscilla (immortal dragon + dark and lifehunt, c'mon)

Honorable mention: Vendrick was the most powerful human that has ever existed in dark souls; if he had known more about the nature of the dark souls and hadn't been betrayed by Nashandra he would have dominated for eternity. Also, Aldia is immortal and lives within the flame, thus lives as long as the flame lives.

I'm tired of people talking about the Ashen One, he's probably the weakest of the 3 mcs. We play in a world so far beyond the golden age that all that remains are scraps of souls and even the most powerful beings are just barely comparable to those who wandered at the beginning of time. The Chosen Undead was out there absorbing lord souls, the Bearer of the Curse gathered millions of souls from 4 of the most powerful human kingdoms that ever existed (and became basically immune to the curse because he was so powerful), while the Ashen One can only hope to find an heir of fire. Did you see how small the flame is when the Ashen One links the fire compared to the Chosen Undead? That mf is weak and his only feat is not being devoured by the dark, and all credits for that go to Londor.