Ive had all three, the 21mm is great if you can abide the aperture, compared to faster lenses-- Im back into Leica again, and trying to decide which 21mm to go with, and Im pretty sure Ill go with the Voigtlander.
As for the 12mm v 15mm issue, I started with the 15mm, and moved to the 12-- I think theyre about on par regarding sharpness, they both have much more light falloff than the 21, which I rather like. The build quality of the 12 is much better, and it is rangerfinder coupled, which is totally useless, given the DOF.
The 12mm is harder to use without just screaming attention to the wide angle effect. 3mm doesnt seem like much, but it is.
The finder in the 12mm is better built as well.
I love wide angles, and rarely use anything as long as 50mm- so I may be a bit biased. However, ther eis no question in my mind, if you are going to have only one of the 3, it should be the 21mm.
15+21, or 12+21 are great, but only one..21.