And don't forget the Summilux, which is stunning. Pity it's $5995...
I own the 21/4 Voigtländer (tiny) and a 21/2.8 Kobalux (also sold as Pasinon and others -- fast enough for most purposes), and have owned or used 21/4.5 Biogons old and new (virtually zero distortion but too slow for my taste), 21/2.8 pre-aspheric Elmarit (excellent but it was stolen from me in Russia), 21/2.8 Biogon, 16-18-21 Tri-Elmar and (not in Leica fit) 21/4 Nikkor. All are very good to excellent so it comes down to price and speed.
If price were no object I'd probably go for the Tri-Elmar (the slow speed is outweighed by having the other focal lengths) and if I used 21mm more I might just go for the Summilux (which I currently have on loan). As it is, the Voigtländer is tiny and so cheap that I carry it without really thinking, and the Kobalux is the lens I use when I really want a 21mm -- but I'm not a great 21mm user.
With 21mm finders, Zeiss are the easy winners.
Of course if you're willing to consider a 24mm (for which, after all, you have a built-in finder) the Summilux is this time the out-and-out winner if you need speed.
Cheers,
R.