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what is the most powerful Cleric domain/dominion in 3.5e?
Definitely a Persistomancer who abuses Divine Metamagic to persist short-term buff spells. Persist Greater Consumptive Field and walk past a pile of ants for near-infinite strength all day (or murder a sack full of kittens in a consumptive field ritual). Use Extraordinary Spell Aim to exclude yourself from your persisted Antimagic field.
Undeath Domain gives you Extra Turning. Planning Domain gives you extend spell. Being a human with two flaws and a positive charisma score means that you'll become a demigod rather early in the game. Not sure which deity grants them both though.