Mechanically it'd be the Illumians of course. They are a versatile Race with some really nice options.
Like Str is trivial to increase to imagine using Aeshkrau to base all your Bonus Spells off Str. Illumians also get access to Still Spell & Divine Metamagic without having to take the Feat. Plus yes, having a Racially granted +2 CL Bonus (for the purposes of duration, sr & such) at the 2nd level really helps Summon durations & buffs.
But Improved Sigil is trollbait amount the weak and retarded and Lesser Aasimar better for none-Prepared Charisma-based Spellcasters, and Charisma is a much more important stat than Intelligence (SLAs/SUs/etc are based off it). So I like them better.