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Linux...as it is open source, you can modify it according to your needs. Many of the available software programs, utilities, and games available on Linux are freeware or open source. br /Linux is and has always been a very secure operating system. Although it still can be attacked when compared to Windows, but still it is much more secure and is not effected by viruses and other malicious codes as you can set the file permissions.br /Out of the box, I'd say certain flavors of Linux, followed by Mac, followed by Windows.
All above are stable. Now it depends on your work and choice. If you are looking for free OS then obviously Linux has many flavours. I will recommend Ubuntu or Fedora. The LTS (Long Term Service) version of Ubuntu will be best one in terms of stability.
Linux