I feel quite defensive about webmin because it helped me so much when I first set up my server, but I am aware of how popular cpanel is, and I have used the user end of it before.
I am however completely unaware of what features cpanel gives you to the server management side of the game.
From what I've seen of directadmin, (i've played with the demo panel) the feature set seemed limited to the very basics.
Virtualmin is a commercial product as well as a free one, with many features only becoming available in the paid product, but virtualmin gpl stands strong on its own. The paid version for example adds an extra tier to virtual hosting, with reseller accounts.
Compare.
The webmin component (can be installed without virtualmin) is a full control panel with quite an array of 3rd party plugins. My webmin is standing at having 120 modules installed. It has its own mysql manager, you can edit disk mapping, network interfaces... It has full modules to control things like bind, sendmail, apache, postfix, ldap... and at least 8 modules managing clustered solutions.