I would say the Mustang, but you can't get a newer (aka better) one for under $15k. The Miata would be a good one if you're okay with not having too much power. Good on gas and great handling. You can easily get a relatively new one (around 2008) for under $15k. Maybe an older BMW 3-Series (though that will cost some money to maintain).
I can't think of much else. Hopefully someone else has some good ideas.