It depends what task you want to do.....generally this is the pros and cons of every OS in the market:
1) Microsoft Windows
Pros:
The largest library of programs and applications.
Some commercial games work only with Windows and DirectX.
Almost all hardware has drivers which are compatible with Windows.
Most widely used.
Cons:
Prone to viruses, spyware, and adware if proper steps not taken such as installation of internet security software, which reduces performance speed.
Requires regular maintenance to avoid system errors and reduced performance.
Even with maintenance, issues can slowly accumulate requiring a re-installation of Windows to restore performance and fix software issues.
Purchasing a Windows license/install disk can be relatively expensive
Windows Vista, the most recent version of Windows, has high system requirements.
2) Mac OS X
Pros:
Relatively simple and intuitive interface with many advanced features.
Not a target for virus or malware attacks, generally: there are no Mac OS X viruses in the wild, and with its UNIX base, it is fairly resilient. There are a small number of trojans (malicious software) which have been known to take effect through applications running in Mac OS X, though they are almost always found on illegal-content-based websites and are not of significant concern. Apple has begun integrating malware blacklists into Mac OS X to prevent infection.
Second-largest selection of software, with many high-quality Mac-only programs in existence, such as those found in iLife. Although at one point software for certain tasks were not Mac-compatible, most software is now either Mac-compatible or has an equivalent that is. Most advanced games have also recently became compatible with Mac OS X.
Reliable and high performance.
Cons:
Limited to Apple-manufactured hardware.
Some hardware does not have Mac-compatible drivers, though the number of this hardware has recently become small and negligible.
In some cases, it may be difficult to do cross-platform network hardware sharing for a Mac OS X based computer in a Windows network or visa-versa. For example, a printer's Mac driver may not support cross-platform printing to a Windows-connected printer, and replacement drivers may or may not exist.
Smaller library of applications in comparison to Windows.
3) Linux and other UNIX-based
Pros:
Low number of viruses and other malware, similar to Mac OS X
Usually free
Large selection of Linux distributions to choose from.
Has a large amount of free and open source software equivalents to commercial Windows and Mac software
More easily customizable
Can run many Windows programs with the use of a compatibility layer such as Wine. (Programs such as Cedega can be used to run many Windows games)
Reliable with good performance and low requirements.
Cons:
Advanced use and installation of some software may require more advanced knowledge than is required for use of Windows and Mac.
Many commercial programs do not support Linux
Small commercial game library
But you can't say the Linux or Mac OS is better from windows (an example) every OS is different form each other and they are designed for different uses and users.