Suggestions for the Cleric List:
Poison: Spell save is 10+1/2 caster lvl + Wis mod., and is delivered with a touch attack.
Spell Immunity: Protects against 1 spell/4 levels, including all magic items, spell-like abilities, etc. that use that spell. Spell must be 4th level or lower.
Righteous Might - the Dragon gets ONE SIZE CLASS LARGER, its damage dice increases, AND it gains +4 strength. For a Huge creature, it becomes gargantuan and (IIRC according to Sage Advice) gains all the strength and CON bonuses that entails (although it gains the penalties to Attacks and AC, too). A Size increase is a HUGE deal for a dragon (no pun intended).
Unholy Aura - +4 to AC and Saves, and SR25 (though many dragons have this SR by this time)
Fire storm - excellent for Fire-immune creatures such as red dragons. If said dragon had the energy substitution feat, then ALL sorts of things could happen here.
Miracle - as has already been mentioned.
Implosion - as has already been mentioned.