There are good choices in all three of those brands. I have lenses of all three and like them all. I do wish Tamron and Sigma didn't make their zoom rings as stiff as they do, but it isn't a deal killer.

Before looking for a better lens, though, it might be worth checking whether your current lens and/or camera have an autofocusing problem. I'd suggest printout out this autofocus test chart to see if you are currently getting correct autofocus:

http://regex.info/blog/photo-tech/focus-chart

If you are currently getting correct autofocus:

For the most bang for buck, I'd look at the Tamron 17-50mm f/2.8. However, there seem to be a high percentage of bad copies of that lens, so you might have to return it for another copy, and you might have to do that more than once:

http://www.bhphotovideo.com/bnh/controller/home?ci=0&shs=pentax+tamron+17-50mm&sb=ps&pn=1&sq=desc&InitialSearch=yes&O=jsp%2Fproductlist.jsp&A=search&Q=*&bhs=t

Another excellent choice would be the Sigma 18-50mm f/2.8. But note, it's really closer to a 19-50mm f/3.0:

(note, don't confuse this lens with the similar-sounding, but mediocre, Sigma 18-50mm f/3.5-5.6, which would be no better than your kit lens)

http://www.bhphotovideo.com/c/product/463426-REG/Sigma_581109_18_50mm_f_2_8_EX_DC.html

The new Pentax 17-70mm f/4.0 is getting excellent reviews, even better than the older Pentax 16-45mm f/4.0, which is also excellent:

http://www.bhphotovideo.com/c/product/567353-USA/Pentax_21740_SMCP_DA_17_70mm_f_4_AL.html

The Pentax 17-70 is a bit pricey because it just came out, but the Pentax 16-45 is excellent too and about US$100 cheaper:

http://www.bhphotovideo.com/c/product/315380-USA/Pentax_21507_SMCP_DA_16_45mm_f_4_0_ED.html

Pentax users are also generally very happy with the Sigma 17-70mm f/2.8-4.5:

http://www.bhphotovideo.com/bnh/controller/home?ci=0&shs=pentax+sigma+17-70mm&sb=ps&pn=1&sq=desc&InitialSearch=yes&O=jsp%2FRootPage.jsp&A=search&Q=*&bhs=t

The Pentax 16-50mm has excellent image quality and is also fully weather-sealed (as is your K20D). But it's also expensive, big and heavy, and like the Tamron 17-50, there have been a lot of bad copies, so you might have to return one or two before getting a good one:

http://www.bhphotovideo.com/c/product/485180-USA/Pentax_21650_SMCP_DA_16_50mm_f_2_8_ED.html

Another option I would suggest seriously considering is an excellent prime (non-zoom), which will not only give you sharpness that only the very best zooms can approach, but will also give you much better bokeh (beautiful out-of-focus backgrounds) than even the very best zooms, and will be smaller and ligher than a zoom, and will let in more light than a zoom. I would suggest looking at the Pentax 35mm f/2.0. Although I have lots of zooms to choose from, I use this lens a lot:

http://www.bhphotovideo.com/c/product/180121-USA/Pentax_22190_Wide_Angle_35mm_f_2_0.html

Here's a good place for reviews:

http://www.photozone.de/all-tests

Popular Photography magazine is also a good place to look for reviews:

http://www.popphoto.com/cameralenses

I also like to look through serious user reviews, and the Fred Miranda site is a good place for Tamron and Sigma lenses:

(whoops, I'm only allowed 10 links in an answer, so here's a non-link)

dub dub dub dot fredmiranda dot com/reviews/index.php

The Pentax Forums website is a good place for user reviews of Pentax lenses:

(whoops, I'm only allowed 10 links in an answer, so here's a non-link)

dub dub dub dot pentaxforums dot com/lensreviews/category-SMC-Pentax-DA-Zoom-Lenses.html

I've never had the kit lens, but I've had the Pentax 16-45 since it came out in 2004 and have been very happy with it. It's very good at f/4.0 and razor sharp by f/5.6.

I recently bought the Tamron 17-50/2.8, mainly to throw unavoidably cluttered backgrounds further out of focus (by using f/2.8 rather than f/4.0), but also to get razor sharp performance by about f/4.0 instead of f/5.6, and thirdly to be able to shoot in half as much light (by using f/2.8 instead of f/4.0) while still getting very good sharpness, about as good as the Pentax 16-45 is at f/4.0.

However, my copy of the Tamron 17-50 had a front-focus problem. I decided to send it into Tamron rather than play games with the retailer, and also to ask them if they could loosen the zoom ring while they were fixing it. I was a bit disappointed when they told me, after I rushed the lens to them, that it would take them 3 – 4 weeks to do the repair. I'm still waiting for it. But for the couple days I had it, I was very pleased with it's handling — it feels just right in my hand while shooting.

My advice —

If you're not in a hurry and don't mind returning the lens a couple times to get a good copy, it's hard to beat the Tamron 17-50mm f/2.8's bang for buck.

For the best likelihood of getting a good copy the first try, and also to avoid an overl
References :
I just realized my answer was truncated. Maybe they'll let me finish in this "source" section.

For the best likelihood of getting a good copy the first try, and also to avoid an overly stiff zoom ring, I'd look at the Pentax 17-70mm f/4.0, or if that is too pricey, the Pentax 16-45mm f/4.0.

If you like the idea of taking it out in the rain with no worries, I'd look at the Pentax 16-50mm f/2.8, if you don't mind the cost, size, weight, and the possibility of having to return one or two to get a good copy.

Good luck and have fun!

Greg