I Concur with what most have said regarding Zeiss 15mm (which I used with M8) and my observations of using WATE and the two 18's (Zeiss & Leica) with M8/M9 for a short bit of time. Most of what's expressed below has been observed by many others *** Keep in mind the one issue I denoted at the end of this post!
Using zone focusing with the 15mm at close range was often hit or miss although through most of its range its optical performance was good and quite sharp except some softness seen in the corners at mid and close distance, as Guy observed. At infinity, it did much better. Only used this one on the M8, so I suppose it will be worse on the M9 and to what effect coding will have, and any possible issues is hard to say.
Of the three remaining lenses it's a toss up and depends on what characteristics is of primary importance although there is one current issue with the M9 and at least one of the lenses.
The Zeiss and Leica 18's had best resistance to flair when shooting into the light, more so than the WATE which was no slouch. As for sharpness across the frame it was close and two samples of the Zeiss I tried, one was sharper than the other, so as it is often with ultra wide angle lenses, proper centering of all those elements can come into play. I did find the one Zeiss ever so slighting sharper in the center of the frame than the Leica...but ever so slight, when pixel peeping, but the slight edge would be given to the Lecia in the corners. The Wate in the center was again ever so slightly behind the other two in the center of the frame, but it's corners slightly beat out the Zeiss. Again one would have to be looking at a 100% crop.
As all others pointed out, the Leica 18 definitely had a warmer rendition that the Zeiss, which is somewhat neutral-clinical, depending on the light. The WATE a very close second (to the Leica 18) in terms of warmth. Of course the versatility (what many find) of the WATE, if so desired, can't be matched by the other two.
I almost got to try a Zeiss 16mm "fixed" f8 (in M mount) with the M8, as someome offered it to me to try, but unfortunately I didn't...although I suppose zone focusing would have also been an issue. Tiny lens!
One cannot beat the Zeiss 18 for price/performance....it's an incredible lens (and value) in my opinion, especially if an 18mm is not a mainstay for the work you do....but Leica as usual upped the ante.
Except for a few short tests with the Wate, I can't say much but Jono and others have sung its praise and new life on the M9 and from images I've seen...impressive!
***Here though is one situation that I'm sure others have heard about and of concern (and please correct me if I am wrong in any way). With the Lecia M9 and Leica 18mm, some have reported magenta infiltration, most notably on the left side and left edge of the frame (much more so than the right, if any). Cause unknown but supposedly will require a firmware fix. Maybe would happen with a Zeiss 18 coded as a Leica 18...but don't know nor heard anything so far. This phenominon not reported on any M8's that I know of. Interestingly on three consecutive frames I shot about 6 months ago using a friends M8.2, I thought I saw something similar using the Zeiss 18 (coded as a WATE & set as an 18mm) with moderate magenta on left side of the frame and didn't think much of it at the time. Now I'm wondering but have no explanation...different lens and different camera, but I remember those frames well, ever since noticing them!
Dave (D&A)