Cain and Lilith. They were the two most powerful in the series, even though Lilith has less durability than Cain, she is still so powerful to rival him in power. After their death, Dean was the most powerful after winning the Mark, as the mark gave him the opportunity to kill every demon in hell.
The only one known to surpass God's near-infinite power was Amara, his twin sister. God has been said by Billie to be more powerful than the Shadow, the ruler of the Empty, who in turn is more powerful than Billie, the new incarnation of Death.