Is OpenSSH for Windows (using Cygwin) is considered to be robust & stable for enterprise use?
You can use freeSSHd. It works fine. No Cygwin required.
I don't see why any other free ssh server solution will outperform a cygwin openssh server on Windows regarding stability. (The installation process may be a little tricky but this doesn't affect stability.)
The cygwin openssh is a direct port of the most used and probably most stable ssh server currently available. I for myself didn't experience any problems yet with all of my cygwin openssh installations on private as well as server machines.
So if you are looking for a free ssh server, why don't give openssh a try?
As for commercial solutions I can't give any recommendation.