It depends on how long you're really intending to continue the campaign. If you're looking long term, the Rainbow Servant is the answer to your question.
A wizard/rainbow servant will, by 15th level, have the spells per day of an 11th level wizard (at caster level 15, with Practiced Spellcaster), and also have on his spell list all cleric spells and those of the Law, Good, and Air domains. He can use wands, staves, and both arcane and divine spell scrolls. He can prepare, for instance, heal as a 6th level wizard spell. Plenty of utility in that build, but it's a long term thing.
Short term, you're probably best off with a druid, but that raises alignment issues for a paladin, and may drop your leadership score. If you can't do that, you're probably best off with either a transmuter or abjurer as noted, but don't overlook the Arcane Disciple feat, which can expand your spell list slightly.
A wizard/rainbow servant will, by 15th level, have the spells per day of an 11th level wizard (at caster level 15, with Practiced Spellcaster), and also have on his spell list all cleric spells and those of the Law, Good, and Air domains. He can use wands, staves, and both arcane and divine spell scrolls. He can prepare, for instance, heal as a 6th level wizard spell. Plenty of utility in that build, but it's a long term thing.
Short term, you're probably best off with a druid, but that raises alignment issues for a paladin, and may drop your leadership score. If you can't do that, you're probably best off with either a transmuter or abjurer as noted, but don't overlook the Arcane Disciple feat, which can expand your spell list slightly.