Spellthief 1/Mystic Fire Knight (Champions of Valor 45) Paladin 5/Unseen Seer 2/Abjurant Champion 1/Knight of the Weave 1/Ultimate Magus 10
Uses Sword of the Arcane Order at 6th to prepare wizard spells in paladin slots, qualifying for prereqs requiring arcane preparation from a spellbook, as well as prepared divine casting. Master Spellthief taken at anytime allows for caster level buffing for paladin casting, and ultimate magus always advances paladin casting as the primary thanks to Knight of the Weave's caster level additions. Battle Blessing allows all paladin spells to be free quickened, while Knight of the Weave spell slots allow for random 1-off spells and ultimate magus metamagic boosting. Use the advanced learning from Unseen Seer to grab Hunter's Eye, PHB2.
Caster level breakdown interpretation that doesn't go infinite:
Paladin: (5 paladin + 2 unseen seer + 1 abjurant champion + 10 ultimate magus)/2 because paladin caster level is halved +2 Mystic Fire knight ability + 4 spellpower + 4 practiced spellcaster = "19", plus KotW's caster level thanks to mystic fire knight making the actual caster level total 50.
Knight of the Weave: 1 Knight + 7 ultimate magus + "19" paladin (Not including the KotW levels, since then it becomes a recursive infinity loop) + 4 spell power = 31
Master spellthief adds all arcane caster levels to any spellthief casting levels to equal the caster level for all arcane spells. 0 Spellthief + 50 Paladin + 31 Knight of the Weave = 81 CL. Any item or ability that grants a + to caster levels, if applicable to both Paladin and Knight of the Weave, nets a +3 gain to caster level. If it only applies to Paladin, it nets a +2 gain to caster level. If it applies to Knight of the Weave only, it nets a +1 gain to caster level.
Granted, this doesn't actually use more than 5 levels of paladin, but it's a paladin of a magic goddess (Mystra, required for KotW and Mystic Fire Knight both) who outcasts wizards from 2.5 spell lists and gets free metamagic, not to mention every spell freely quickened if you use Battlecasting for your paladin slots.