DA15mm Ltd. (I know that you listed FF, but the DA15mm on the K-7 is a great combo IMHO).I have a K-7 and K-3, and continue to use my K-7. For travel, the DA15mm Ltd is a fantastic lens for landscape and city views.
I share some experience of my DA15mm Ltd in comparison with other primes at:
Primes in Paris - PentaxForums.com
I just come back from a trip in Europe, where I brought the DA18-270mm, DA15mm Ltd and FA77mm Ltd. Again the DA15mm Ltd was great. Highly recommended.
My 5 cents, hoping that it may help.