As you didn't mention a specific game, it would be kind of hard to help you with anything tied to a specific game.
In regards to your question about the operating system, Linux / Windows / Mac, you will always end up getting a flame war between Linux lovers, Windows lovers and Mac lovers with such a question. Everyone will always say the one that they love is the best, even if anything else gives a better performance.
However, if you want any kind of "statistical data", then you can see in
this thread, where I posted a operating system scan of the Top 50 servers at
www.gametracker.com for several games ~5 months ago.
The scan revealed that the Linux OS was housing ~
70-88% of all servers running CS:S, CS:GO and TF2 at that time.
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