My vote is for the 35/2. Have owned the 35, 50/2 M and 100/2 M (all used on 1Ds2 and 5D) and my fave was the 35. Sharp as hell starting WO, nice bokeh, not as prone to fringing as the 100. From LC's tests, the 28 does have strong curvature and have also seen some nasty CA off the close-focusing 25 in sample shots from Nikon bodies.
I found the 50/2 ZF under-whelming (my cheaper Leica 60 macro was as sharp or sharper starting @ F4 and far less fringing off reflective surfaces).
Get LC's ZF review/test - well worth it if looking at any ZF glass. Leica lenses I've owned and would I'd strongly recommend as ZF alternatives (check can be converted w/Leitax): 28/2.8 latest (a knockout at any distances and has floating element), 35/2 and 60 Macro. The 50/2 is also nice, but you have that covered. The older Leica 35/2.8 is also a very nice lens (from what I've seen and heard) and a real bargain.