Darth Credence mentioned that the Mark of Handling has the most non overlapping spells in terms of race, in terms of background I think that Golgari Agent is your best bet. So at the end of the day you're looking at Mark of Handling, Golgari Agent. Theurge Wizard 17 Twilight Cleric 3 (Most non overlapping domain spells). That'll get you access to all the wizard spells, all the first and second level cleric spells, the twilight cleric domain spells, about 14 extra spells from your background and then a about 10 3rd level or higher cleric spells. Pick up a couple of bard and warlock only 1st and 2nd level spells via feats and you'll have everything save for a couple like Glibness and Find Greater Steed. But I guess you can just use Wish if you really need them.