Which is your favourite child?
DA 12-24
DA 18-135 (here at 21mm)
DA 20-40 Ltd (here at 20mm)
DA 35 f2.4
FA 43 Ltd
DA 50 f1.8
FA 50 f2.8 macro
DA-L 55-300
DA 55-300 PLM (images at 70mm, stitched)
FA 77 Ltd (stitched pano)
DFA 100 macro WR
FA*300 f4.5
Sigma 400 f5.6 tele macro
As you would expect, I use the wider ones (12-24, 18-135, 20-40) the most for landscape. The 12-24 and 18-135 are both very good, although they would be better if they had HD coatings (both susceptible to purple fringing in high contrast areas, and the 12-24 to flare).
I love the Limited look from the 20-40. It is particularly good at the wide end (where it has the bonus of f2.8 for occasions when it's required). To me it's a DA 20mm f2.8 Ltd with 19 other focal length thrown in for free. It's not the absolute sharpest lens (the DA 35, for example, is sharper when I compare them at 35mm), but it is plenty sharp enough. What makes it special is that can produce images that are contrasty, have excellent colours and gorgeous overall rendering. (I haven't used the DA 21 Ltd, but I suspect it would be similar.)
All the telephoto lenses can do good landscapes, but the 100 macro is the one I enjoy the most. It produces superb colours and contrast, it's sharp at every aperture and it's sharp corner to corner from f5.6 to f11 without being boring.