This is indeed a tough question, and the answer rests with you! What do YOU want, and what are your acceptable standards in a lens.
I bought my K10D with both DA kit lenses, and have since sold them in favour of other quality lenses. I first purchased the Sigma 18-200 DC and while it is fine for scenary and people shots, it has a problem with architectural shots. I learned to live with the wide distortion by purchasing 'PT Lens' which does a fine job of straightening the distortion, and this lens does a great job for what it is.
I now mostly use the Sigma 17-70 as a walk around lens, and keep the 18-200 for back up and long shots when I do not have the Bigma 50-500 with me.
I have been thinking of the Tamron 18-250 and have seen some good reports and reviews on this, but it will not come close to the Pentax 16-45 or Sigma 17-70, but I do not think it is much better than the 18-200.
These example shots at the links below, were all taken with the Sigma 18-200, if you are happy with this quality, which I am for a walk around lens, then it is a good lens for the right price.
Phil
Some 18-200 shots:
Storm on the plains A Sunday Drive in January