suggestions for first pentax prime lens

I own lots of Pentax zooms, but I'm looking for a suggestion for a first pentax autofocus prime. What is your favorite Pentax fixed focal length lens?

I own a Pentax 16-45mm, 10-17mm fisheye, 55-300mm, and the 18-55mm kit lens, along with the K-x camera. I also own a few third party lenses -- the Sigma 30mm F/1.4, Tamron 28-75mm F/2.8, and the Sigma 150-500mm, and a manual focus 50mm F/1.7 and a manual focus 55mm F/1.8 m42.

• Shel, my photos, http://www.sheltx.com/