You already got a lot of good advice. You cannot go wrong with the FA31mm and FA77mm, but, more generally, you cannot go wrong with the Pentax FA Limited and Pentax DA Limited lenses. These lenses are great, great, great....
In your final decision, I recommend that you consider how you use the lenses. Personally I like to carry the lens with my, in a small carrybag on my shoulder or even on the camera strap around my neck. For such an use, you need a small camera+lens system and weight is always an issue. As Adam pointed accurately, the Pentax FA Ltd and DA Ltd are nibbles. They are great for their compact size and relatively light weight. (In contrast, the Sigma lenses tend to be heavier and bulkier.) If you can to to a brick-and-mortar shop with your camera and try some lenses on. You will see the difference and it may be revealing....
Among my favourite walk-around primes, let me share my praise for 4 lenses:
Pentax DA15 mm f4 Ltd - A great, great wide angle lens - In-depth review at
Pentax-DA 15mm F4 Limited Review - Introduction - PentaxForums.com (score 8.8/10)
Pentax FA31mm f1.8 Ltd - An outstanding IQ, the perfect all-around focal length for APS-C - in-depth review at
Pentax-FA 31mm F1.8 Limited Review - Introduction - PentaxForums.com (score 9.3/10)
Voitglander Nokton 58mm f1.4 - Built like a tank, superb IQ, excellent in poor weather light conditions - Highly Recommended by Photozone
Voigtlander Nokton 58mm f/1.4 SL II - Review / Test Report
Pentax FA77 f1.8 Ltd - Great for portrait, very nice IQ - In-depth review at
DA 70mm vs FA 77mm Limited - Comparison: Introduction - PentaxForums.com (score 9.2/10)
Each lens is different but all of the above are relatively small and light-weight, although they are all metallic construction.
Hope that the comments may help.