Lots of different replies
Yes a zoom is more convenient and only having 1 prime as the only lens is limiting. I use primes but then I have several so I have choices.
But, if there was to be only 1. Then the traditional all around lens is around 50mm. Which means on pentax dSLR it would be around 35mm.
Which makes the FA 35mm f2 a great choice. I use this one a lot. I wouldnt get anything longer. But the FA 31mm I think for me would be even better in terms of focal length since its slightly wider.
Next would be the DA 21, a nice lens, but that is getting quite wide but depending on your shooting style and preference that could be it.
So I would go for either the FA 35 or DA 21 depending on how wide you like it.