Hello,
Cnprog appears to be inactive, but is still strong in the form of askbot. OSQA is also a clone of CNPROG. I am not sure how active is osqa development, seems to have slowed.
In terms of bugs fixed Askbot is the best (from the three you've mentioned), I keep bugs out by writing test cases for every new feature.
You can monitor project activity and history here: http://github.com/ASKBOT/askbot-devel/network. Besides my own there are some contributions by several other people - mostly translations at this point, but forks are welcome.
Other projects that I'd mention are Shapado (Ruby/Rails) and Question2Answer (PHP). Maybe there are other decent ones, but I don't have time to research - who will tend to askbot then :)?
Thanks.