I'd either get the Leica 28/2, the 28/2.8 Asph, the ZM 28/2.8, or the CV 28/3.5. The Leica 28/2 is awesome. Fast, sharp, relatively compact, expensive. If you want something cheaper, go for one of the other 3 options.
THe 28/2.8 Asph is relatively affordable for a Leica lens, is small and sharp. I'd take it any day over the 28/2.8 IV, just for its size. And for not much more than the IV, it's brand new too.
The Zeiss ZM 28/2.8 looks like a nice lens too. It might not be as sharp, etc., but I've seen great pictures from it. Reasonably priced, especially used. I had no problem with my 35/2 when I owned it.
The best buy is the CV 28/3.5. The cheapest of the bunch and one of the smallest M-mount lenses made. It's very well constructed too. Very sharp, but a bit slower than the rest.
I have the 28/2 and the CV 28/3.5 and both get used. I'd love to have the 28/2.8 Asph, but obviously can't justify it while owning 2 other 28's. Maybe if I hit the lottery.