I'm not nearly as good as most people here at landscapes, and I certainly don't shoot as much as I would like. But I seem to "click" with the DA 16-45mm f4. A boring choice, I know, a cheap lens from the early days of APS-C (released back when the *istD was the only Pentax DSLR in existence...)
But I like what I get from it.
As for other lenses, I also enjoy the DA 35 2.4 "Plastic Fantastic"... even though it also gets a bad rep. It's very sharp at infinity and I think the colors and contrast are great and respond very well to PP.
The M 28 3.5 has gorgeous colors, and the K 28 3.5 is supposed to be even better. I have the M, and should use it more.
I've used the DA 21 before, I had one for two weeks on a trade with the DA 16-45 and then the trade was undone, as the other person decided it was decentered when used in portrait orientation only - I haven't seen that, and it's possible that it happens because of the barrel wobble that the 16-45 is famous for. If it does it would probably happen at the wide end - but I never use it in portrait orientation at 16mm, so I wasn't going to argue with it, and I was actually glad to get my 16-45 back as I really like the lens.
Having said that, I'm thinking about swapping my Tokina 19-35, which early on did give me some nice landscape images when it was all I had, for a DA 21. I was holding on to the Tokina in hopes of winning a K-1 this summer... but obviously that didn't happen. So sometime next year I think, I'll have a DA 21 again and I'll give it another chance...