I have the 12-24, the 10-17 and the 21ltd (in order of acquisition). no experience with the 15mm.
I added links to my set with these lenses, so you can see what the same photographer can do with these lenses.
Pentax DA 12-24mm f4.0 ed al if - a set on Flickr Pentax Fisheye 10-17mm f3.5-f4.5 Zoom - a set on Flickr smc PENTAX-DA 21mm F3.2 AL Limited - a set on Flickr
For you I would recommend the 12-24. It's a very competent lens, and the most versatile of the three. I think it's an advantage to have a zoom when shooting wide-angle, even more than for the larger focal lengths. That is because a lot of the background will be in the picture, and mostly (almost) in focus as well. So it helps if you can zoom a bit in or out, to get the maximum effect, with zooming in to just get a disturbing object just outside of the frame. Maybe that's the reason why I use this lens at all focal lengths, by the way, not only the extremes (although a bit more at 12 and 14/15 than the rest).
I decided for a 12-24 and not a 10-20, because I think the 'distortion' of rectalineair lenses wider than 12 is too much (I mean the stretching out of object, necessary to keep the lines straight). For views wider than 100% (12 mm) I prefer the fisheye. But that's very personal.
I hope this helps to make up your mind. And, if you can't choose: get 'em all :-)
---------- Post added 03-25-2014 at 10:04 AM ----------
oh, as for CA on the 12-24mm: that's a weak point of this lens, in some circumstances clearly visible (and the 10-17 mm even more). But not so difficult to erase in lightroom.