I'd say the single most potent attack we've encountered is still Ultima. We've encountered beings stronger than Ultima weapon such as Omega, Hades, Alexander but they tended to demonstrate their phenomenal cosmic power in other ways than a brute force attack against us. The Ultima spell is still the only one that required Hydaelyn to protect us.

In a no holds barred gauntlet I'd say Alexander. Its hard to beat something that sees through time and manipulates it at will.