Ill also cast a vote for the CV 15mm
Its drawbacks are mainly a high degree of vignetting, the use of a viewfinder instead of through the lens (my other 15mm for SLR gets very different, arguably better, shots just for that reason) and that it's at its best between f/5.6 and f/6.3....
The old Tokina/Vivitar 17mm f/3.5 is surprisingly good (and much better than the overrated Tamron 17mm). Its main deficiencies compared to the likes of Zeiss & co. is probably more tendency to flare and a bit more distortion.
The "classic" Zeiss 18mm has a great (3D) look to it. Corners could be sharper though. Still a very satisfying lens (I much prefer it to the Vivitar 17mm, even if the sharpness isn't necesarily better).
Leica 19mm (II) is a great lens, but not that different a FL from your 21mm. Having such lenses on an SLR could make a difference in use though.
The Zenitar 16mm fisheye is actually very good, if you want to go down that road. I compared it to my Zeiss 16mm: It was actually *slightly* sharper in the centre and *slightly* softer at the edges.