If we have to pay for it, it had better be stable and complete.
But seriously, I would have to say that CentOS is stable and complete, and free. It's supposed to be an open-source OS for corporations, so they're really working their tails off to keep it secure and reliable, and my experience with CentOS certainly proves that they've done their jobs well.
Well you can consider all the usual suspects for a stable linux distribution.
Open Source/Free: Debian, CentOS, Slackware
Commercial: RHEL, SLES (SuSE Linux Enterprise Server).
Although Slackware isn't targeted as an enterprise distribution, in the hands of a good admin it will work wonders for you (albeit with a bit more legwork).
But remember, a distribution will only stay stable and secure if the admin 1) knows what he/she's doing, 2) keeps alert to new security vulnerabilities and 3) doesn't keep tinkering with the server until something breaks
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