Proponents of each operating system will always give you different answers to this question, but it is important for you to decide which one is best for your circumstances. Choices include the many variations of GNU/Linux, the numerous descendants of BSD, several versions of Windows Server, Solaris, Mac OS X Server, and several proprietary iterations of UNIX.
The following are some of the points you should consider when choosing an operating system.
1. Cost – Like purchasing a vehicle, house, or other major expense, you should view more than just the price tag on your operating system. You should look at total cost of ownership (TCO). How much will it cost you to maintain it or pay someone to maintain it? How much will upgrades cost? How much will you spend fixing the problems it creates?
2. Longevity – You need an OS that will be around for a long time with frequent security and stability updates.
3. Support – How much support do you need? If you have a full IT staff, you may be perfectly comfortable managing a free version of Linux like CentOS. Otherwise, you will want some type of support licensing for Linux, Windows, or whichever OS you choose.
4. Security – Granted, an OS is only as secure as you make it, but some are a lot easier to make secure than others. You should evaluate them and determine which is best for the type of server you want to run.
5. Flexibility – If you want the ultimate customization, a free and open source OS like Debian GNU/Linux or FreeBSD is probably best. It all depends on your needs, so choose wisely.
6. Software needs – If you need a very specific software stack for your web applications, that will definitely help you decide which OS you need. While some software is cross platform, it may work better on one OS over another.