Linux (specifically Ubuntu, CentOS and SUSE are preferred) as each has distinct strengths but mostly because clients ask for the package management system they are familiar with.
We've always liked Linux more than we publically admitted assuming brand loyalty to Microsoft (even if exagerated) wuold be helpful - loyalty to Microsoft has proven useless so why hide the facts?
We've ALWAYS had a lot of critical items on Linux - our dns infrastructure (always from 1995), a large % of our email systems (always, since 1995), several monitoring apps, our support tracking system (apache + perl + mysql) and more.
Since we've managed such critical applications on Linux infrastructure for over a dozen years we can handle any needs you have on Linux.