It depends.
For netbooks, PC, because Apple doesn't make one (the MacBook Air doesn't count as a netbook). I'm not a huge fan of netbooks anyway, though. If it comes with Windows, though, I immediately ditch it and put Xubuntu on it, or OSx86 if it'll work with the hardware.
For laptops, Mac, hands down. I've had a MacBook Pro for almost 3 years and Apple is absolutely the best laptop manufacturer our there in my opinion, and no OS comes close to all the features OS X has.
For mid-range dekstops, PC again. I build my own, though, and dual-boot Ubuntu and OSx86. I've never seen a pre-built desktop PC that even came close to being exactly what I wanted.
For uber-powerful PCs, 8-core Nehalem Mac Pro w/ 8 GB of RAM. I'd probably have Linux installed on it through Parallels Desktop or VirtualBox, though.
If you're asking about OS X vs. Windows, though, there's no contest in my mind. OS X is faster, lighter, more stable, has fewer bugs and security holes, is easier to use, is less bloated, and doesn't demand the latest hardware just to run. If you want proof, compare Leopard running on an 867 MHz G4 w/ 512 MB of RAM to Vista running on a 1.6 GHz Pentium 4 w/ 512 MB of RAM. I guarantee you Leopard will smoke Vista in any test you throw at it, even with the Pentium running almost twice as fast as the G4. You actually wouldn't even be able to install any version of Vista except Home Basic since every other version needs at least 1 GB of RAM.