PC (as in Windows) - Great value, great potential for being epic win, and great performance.
Mac - Overpriced, Good performance with programs that don't need too much of the systems capability.
Linux (as in 2.6.2 or Ubuntu) - Great value (as is anything free), Great performance, however you need to be a total geek to master it.
My personal preference is Windows 7, and I say this after using Win95, 98, ME, XP, Vista, MacOSx, Linux 2.6.2, Ubuntu, and Xubuntu.
Also, in order to really compare the OS's you have to try them all on the same system specs. Clearly, a 64bit Win7 Ultimate with 8 gigs of RAM is going to rape a MacBook Air, which commonly has only 2gigs.