Linux.
It's faster than Windows, with lower system requirements.
It looks better than Windows or Mac (Compiz -
http://youtube.com/watch?v=7ye5qpt10CU).
It is free.
It has more applications than Windows availible through one click - over 20,000 free and functional applications can be downloaded and installed through one click in about 10 seconds. No more installers.
No viruses at all. No malware.
It has many Mac-specific features (in fact, it invented most of them). "Spaces" and instant search were in Linux before Mac OS X or Vista. There are also Compiz plugins for cool window features like Expose.
Equivalents for every major Windows application that are compatible with the Windows equivalent.
OpenOffice.org compatible with MS Office
Evolution compatible with MS Exchange and Outlook
Firefox
VLC Media Player plays every file format I can throw at it (more than WMP can)
Pidgin (compatible with MSN)
GIMP (compatible with Photoshop)
Working Java (lol Mac Java is broken)
Professional Video editors like Kdenlive for free
Professional Audio editors like Audacity for free
Transmission Bittorent
Inkscape is better than Adobe Illustrator
It also runs Windows games at full speed and compatibility using Wine - Blockland runs faster than on Windows for me.