My most fun character in 3.5 was/is my Paladin of Freedom(fallen)/Wizard/Ur-Priest/Mystic Theurge.
Never ran out of spells and still took the Stormbolt Reserve Feat just to be add insult to injury.
Another facet of the fun was allowing my fellow players to believe the character was a Paladin of Freedom/Wizard/Cleric/Mystic Theurge. Suspicions grew with the rapid divine spell progression and later the Ur-Priest part was confirmed when the character started lobbing Vile Lightning Bolts.
Thanks,
Rich