Risen Martyr without Final Ascension.
It looks like the whole "can't take levels in other classes" thing is part of Final Ascension, so...
It's not that bad of a class once you've stripped out that bit of idiocy.
Another one:
Warhulk without No Time To Think.
EDIT: The Auspician from Faiths and Pantheons is an awesome PrC.
On the one hand, it has really shitty prerequisites; you need Dodge, all of the save boosting feats (Iron Will and company), need to be Chaotic, have the Luck domain, Fort +5, and Ref +2.
In exchange... it's a 10/10 spellcasting PrC with good Fort and Will, a d8 HD, and a medium BAB.
But that's not all!
At 1st level, you get the Fate domain if you don't already have it, and can give yourself a +20 Luck bonus to an attack roll 1/day as a free action. This eventually scales to a +30.
At 3rd level, you can reroll other people's rolls once per day.
At 4th level, you get to pick one option from a list of buffs each time you prepare your spells - notable ones are a +2 Luck bonus to all your spell save DCs, a +4 Luck bonus to Initiative, or a +2 Luck bonus to AC.
At 7th level, you can lend one of those buffs to an ally, in exchange for taking an equivalent penalty yourself - just give the Fighter a +1 Luck bonus to melee attack rolls or something.
At 8th level, you gain one option off the list of buffs as an Extraordinary ability.
At 10th level, you can automatically succeed on one saving throw per day.
The best part? There's no prereq saying what type of spellcasting you need, and you just need the Luck domain to get in. So Arcane Disciple comes to the rescue!