I agree on the sigma 12-24mm. If you want a uwa zoom, it is the best one for under 1k.
Unless you need the flexibility of a zoom, I would recommend a fixed focal length for anything under 24mm. I recommend this lens a lot (too much maybe), but the Samyang/Bower/Rokinon 14mm F2.8 lens is magical. It does suffer distortion, but is easily corrected using a lens profile in lightroom. Many who own this lens create and give out their lens profiles via the internet, which work really well.
Plus, I picked up the lens for $300. Amazing lens for the price