Eternity considering how powerful he's supposed to be. Has he ever won a fight? His sole purpose for existing seems to be to emphasize how powerful some new cosmic threat is.
Seriously, this guy should be on par with Michael, Lucifer, and The Word, considering the fact that he is The Presence's Wrath manifest.
Pathetic CIS limitations by writers (ie. can't judge Joker, can't kill Soulless beings, etc.)
He fought Parallax and GEB (even though he lost, GEB is pretty much soulless). Why can't he fight Nekron?
Special mention:
The Radiant
Phantom Stranger
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"Farewell, Damos... Ash, Pikachu... And you. All of my beloved." -- Arceus