For gaming, it's all about Windows ME, 98, etc.
For print publishing, the Mac is the best.
For development, it used to be OpenStep, NeXTStep, but I would have to say Microsoft makes some excellent development suites. I really like the Visual development packages they have made.
For a server, I would say some BSD variant.
For an embedded OS, Linux is pretty durned good, though I hear Be is really going to push Stinger to the forefront if they have anything to say about it.
And for e-commerce/SAN, databases - Solaris hands down.