Which Lense Is there one must have?

Hi novice photographer who has been having loads of fun learning and playing with my camera. Im hooked.
I have a K10d with kit 18 55 pentax lense a 70-200 f zoom and a sigma 28-70 zoom. I have used the 18 to 55 the most but found that I would like to be able to use a bigger appeture than available. I havent yet discovered exactley what I want to photograph the most yet but I do seem to be interested in architecture, moody scapes,and reflections. but nothing is set in stone. Id love to know if anyone has any suggestions on which lens I should think about adding to the above list. I know that depends on my budget but a good quality all rounder that will serve for years is worth paying for but which one ? Any advice or discussion would be welcomed.

Well the architecture probably is

Ayyway, sounds like you need a Pentax DA 12-24mm or Sigma 10-20mm

How much do you have in the way of funds? Sorry for the directness

I agree with the suggestion. You need an ultra wide angle lens, so have a look at the Pentax DA 12-24mm (if you find the Samsung version of this lens, it's about 40% cheaper!) or the Sigma 10-20mm are good lenses to get. I have the Sigma on my Canon and it's an excellent lens.

Instead of one all-rounder (which will have more distortion than you might want at the wide end), how about an ultra wide-angle such as Pentax/Samsung 12-24mm or Sigma 10-20mm, plus a couple of primes, perhaps a 28 and 50mm?

One or more of these could be second-hand to reduce the cost.

Budget up to about 300 ukp at this stage dont mind you asking. Thanks for all your quick responses.. Will look at all your suggestions.

The wide angle zoom suggestions are spot on of course but you may also consider something like the DA* 16-50 which is more of an all rounder and has a faster f/2.8 aperture. Other options to consider the DA 17-70 f/4, DA 16-45 f/4 (Mike-P has one for sale now) and similar lenses from Sigma and Tamron.
I find my 16-50 is pretty wide at 16mm and a little perspective correction in PP always helps

Looks like I got to up my budget abit then ..
But on that If I had a bigger budget of say 1200 ukp would that alter your recommendations? I can dream ...

OK so at £300 for an all-rounder you're in S/H DA 17-70 or new / S/H DA 16-45 territory or perhaps a new Tamron SP 17-50 f/2.8

The wide angle zooms are a bit more costly still.

Or a well cared for 35 Ltd?

For a 50mm look for the Pentax-A 50mm f1.7, very fast, sharp lens. As it has an A setting you can use all the modes on your camera, although it's manual focus.

For a 28mm consider the Pentax-M 28mm f3.5 - a bit slower but still very sharp and compact. Only snag is that you'll need to learn how to use manual lenses as it doesn't have an A setting, but that's not too hard. See my thread about the lens for a sample photo and crop from it.

Purchased a Pentax-A 28mm f2.8 and Pentax-A 50mm f1.7 about a week ago, and I can wholeheartedly recommend them, the two focal ranges seem to complement each other very well.

Thanks for all of advice.. youve helped me no end ..I was bamboozled by options out there, youve all narrowed it down nicley for me. Now if anyone knows where a nice kindly bank manager with too much cash in his wallet lives Ill be sorted.

Well I was pretty sure to start with but since reading this thread I'm totally bamboozled now.

12-24mm
10-20mm
17-50mm
16-45mm
16-50mm
17-70mm
28mm
35mm
50mm

OK you are all going to shoot me down in flames, but the sigma 10-20 is the worst lens you could buy for architecture, it stretches at 10mm and at 20 it might just work, I swapped mine for the 16-45 and that gives you the range you would need for architecture but you would be better with a prime lens like a 28mm a 30mm or 40mm, the 16-45 gives you all the options so I would go for that as I did.
Jack.

Edit and it's a good price and Mike-p has one for sale at a great price.