He's not the most powerful, but Red Mage from 8-Bit Theater did have a spell capable of freezing the universe, Ice9. It also froze more treasure than actually exists, when cast into a bag of infinite holding.
Been a while since I read it, but I'm pretty sure the bag contained a larger than infinity dimension, as it could only be overloaded by storing things that don't actually exist. And it did have room for them for a while.
Ein Sof
/thread.
But I would say Gregory West from the Twilight Zone. I even made a post on VS Battles arguing for his omnipotence, but they didn't care, because that site sucks ass, so I'm posting it here.