Outsiders with prepared spellcasting?

[RoboEmperor]

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[Malimar]

Ghaele eladrin (casts as 14th level cleric, 10 HD) comes to mind.

[unseenmage]

Ghaele eladrin (casts as 14th level cleric, 10 HD) comes to mind.

And IIRC this one's on the Summon Monster list too

[Fuzzy McCoy]

Trumpet Archons are very similar to Ghaeles, but with 12hd instead of 10. Same 14th cleric casting though.

[Psyren]

Pathfinder added a bunch more if you're interested (e.g. Aeons.)

[ShurikVch]

Guardinal, Ursinal (Book of Exalted Deeds) - as 12th-level Wizard

How about the unique Outsiders?
Asmodeus casts as 20th-level Cleric;
Mephistopheles - as 15th-level Wizard;
Fiend-Sage of Rel Astra (unique Molydeus, Dungeon #150) - as 11th-level Wizard;
Killjoy (efreeti, Return to White Plume Mountian (https://en.wikipedia.org/wiki/Return_to_White_Plume_Mountain)) - as 15th-level Wizard;
Orcus, seemingly, gets new variant of spellcasting on every next statblock: notably - 20th-level Sorcerer in the Savage Tide (https://en.wikipedia.org/wiki/Savage_Tide), 13th-level Wizard-Necromancer (with Divination as prohibited school) in the Book of Vile Darkness, and in the "Headless" adventure (Dungeon #089) - as 20th-level Cleric with Chaos, Death, Destruction, and Evil domains, and 20th-level Wizard-Necromancer (no prohibited school)

[Troacctid]

Here's the reference I bookmarked for stuff like this. http://minmaxforum.com/index.php?topic=1582.0

[Arcanist]

Trumpet Archons are very similar to Ghaeles, but with 12hd instead of 10. Same 14th cleric casting though.

Minor aside, the Ghaeles is objectively better by exactly one point of wisdom.

Get Planar Ally on your spell list, cast Planar Ally, give them 10,000gp for a minimum of 11 days of service. Have them channel through you (Book of Exalted Deeds) to gain access to 14th level Cleric spells for their entire service. To really make this work, you'll need at least 17 Wisdom to fully benefit from your new spell list.