I have been looking for the 'best zoom' lens. I don't have the sigma or Pentax 40..
I used to have one of the best Nikkor zoom, 24-70 f2.8. However, I can get better image quality (sharper) with Voigtlander 40mm f1.4 prime than the Nikkor zoom at 1/5 the cost under good light.
I have only been using prime lenses since i have sold my Nikkor.
Zoom lens has the convenience of not having to swap lens on occasions like wedding, etc.
If you are after the best IQ and large aperture, go for the prime, preferably the prime with the biggest aperture you can afford. f2.8 is not fast enough indoors but the K5 can compensate with higher ISO. I can always zoom with the 'best zoom'- my legs.
Here is a snapshot taken 2 hours ago- K5 with Sigma f1.4 taken at f2.2 indoor at night..
All the best!