In the Forgotten Realms, probably Mystra. If I'm living in a world where magic is that real and quantifiable, I'd probably end up with them. Lathander would be good too: Dawn, the Sun, Life and Healing, all very good things and things I feel strongly about.
If in Dragonlance, I'd probably end up as a Mystic, and owe no specific allegiance to a deity. The Krynnish gods leave and return so often that their crystal sphere practically has a revolving door, better to rely on the divinity within. That and I think the Mystic class is cool and should be in the core rules (they really need a core spontaneous divine caster).
On Greyhawk, probably Pelor, for much the same reasons as Lathander above (I think at one point they even said that Pelorites visiting the Realms get their spells granted by Lathander, and vice versa, and that the two deities quietly worked out such a deal and are on very good terms on the Outer Planes)
Among the Olympian deities, I would certainly say it would be Athena: A deity of Knowledge, Civilization, and Tactics.