Cleric(3)/Wizard(3)/MTheruge(10)/Heirophant(4) - take Spell Power with all four levels of Heirophant. Cast as a 13th level Wizard/21st level Cleric - prepare as a Wiz(13)/Cle(17). ^_^ Or replace the Heirophant levels with Archmage levels, and swap caster levels between Wizard and Cleric.
Or - and this one's a trip - Wiz(3)/Cle(3)/MThe(10)/Archmage(2)/Heirophant(2). Spellpower for both levels of Archmage, and the same for both levels of Heirophant. Cast spells a 17/17, prepare them as a 15th/15th. Eventually, push both Archmage and Hairophant to 5th level, still going the max for spellpower, and (at 26th level), have a caster level of 23/23, and prepare spells at 18th/18th levels.
I currently have (for the Exodus, so it's epic) a Cleric(3)/Wizard(3)/Mystic Theurge(9)/Geomancer(10). 8 of the GeoM levels went to Wizard casting, and 2 of them to Cleric casting; he casts as Wizard(20)/Cleric(14), and has Versatility IX ... letting him sling Wizard spells in full heavy armor, with no ASF%. ^_^ No clue yet if he's a winner, or just a speed bump, though.