80% of the time, my Pentax is wearing a Sigma 24-135 2.8-4.5.
10% of the time, if I need a little wider view, or if I want to have a lighter combo to carry, I put on the Pentax 18-35 FAJ.
5% of the time, I like to put on older primes. My favourite is the 55mm 1.8 Super Takumar (M42 mount with K-mount adapter.)
5% it's something else (Pentax 28-90 FA, Sigma 70-210 UCII, etc.)
You will note that I shoot both digital and film, and would LOVE a Pentax full-frame DSLR. So I try to only buy full-frame compatible lenses.