You already have 3 good 50mm lenses so no need to add another in that range. I guess it comes down to what you like to shoot (and of course budget).
You don't have anything wider than 18mm. I like wide angle so that's the first lens *I* would add. In my case I bought the Tamron 10-24 (for versatility) and later Samyang 14mm (for sharpness wide open for astrophotography). The DA15 and Sigma zooms are also good choices.
There's also no macro in your collection. The DFA 100mm WR or a used non-WR 100 (same optics) are compact and very sharp.
3rd lens (yes, you said 1 or 2, but prioritize based on what *you* like to shoot) would be a longer zoom. I am happy with my Sigma 50-500 OS. It's heavy but the ability to do a 50mm landscape in the middle of 500mm wildlife shots is very nice. The DA 300 plus 1.4 TC (which I don't have) is lighter and reportedly sharper than the Sigma, but then you might need to carry your K200D as a 2nd body adding to the total weight.
It all comes down to: how do your current lenses limit your photography?
---------- Post added 05-01-14 at 11:13 AM ----------
Originally posted by blackcloudbrew Personally, I'd focus on replacing the kit lens (the DA 18-55). Options would be the DA*16-50 f2.8 or Tamron 17-50? f2.8. You might also look at the DA 10-17 fisheye as it's a really great lens for ultra wide angle.
The 16-50 is another excellent suggestion, especially if your current 18-55 is not a WR version. It's liberating to shoot in rain and snow with less worry about ruining equipment.