Best OS for what exactly?
Games: Windows XP.
Anything else: Unix (meant to include Linux, BSD, and others)
Price: Unix.
Security: OpenBSD or Hardened Linux
Malware resistance: Unix.
Ease of use: Ubuntu, SUSE
User power: Unix.
Embedded: Linux.
Efficiency: Unix.
Tinkering: Unix.
Portability: Unix.
Flexibility: Unix.
Compatibility: Linux (yes, better than Windows)
Freedom: BSD
Standards compliance: Unix
Reliability: FreeBSD
(Proof?)
64-bit Support: Linux.
Server: Unix
Transferability of OS knowledge: Shared between Unices. (One who knows Linux will need only a little adjustment to use OpenBSD or FreeBSD or Minix or Solaris or...)
Availablility of technical support professionals: Tie.
Office tasks: Tie.
Web browsing, music, IM: Tie.
Gaming, however, is really the only thing anyone ever actually does with their computer, thus it is a clear victory in this case, for Windows XP. After all, there are no games for Linux whatsoever, and no Windows games run in Linux, making it inviable as an operating system for general-purpose use.