What is the best Mac operating system?
I'm not sure what answer you are looking for but as for Mac OS it's always a wise decision to use the latest one.
It is a good idea to wait a week or two before updating to the latest release simply to see and read about possible issues which may be introduced.
For example 10.6 introduced some incompatibility with older Fiery drivers, 10.6.5 introduced several problems with landscape printing and 10.6.8 again introduced problems with printing ~ one could get the feeling that Apple doesn't quite manage to make proper CUPS adjustments.
Other then that I personally feel that updating Mac OS X is always a good idea. Administrating hundreds of Mac's I always aim for the latest one even if the Mac was shipped with a previous release ~ also known issues never applied to more then a handful of them.
Last but not least you will see developers dropping support for older systems which at some point will force you to upgrade unless you find an alternative App.
If you are looking for an alternate operating system I don't see any profit in that.
Sure, running Windows on your Mac (via Boot Camp or standalone) is a nice treat because you will get all necessary drivers and updates direct from Apple but that's about it.
Unless you really need the direct hardware performance for Games or encoding and stuff I personally prefer using VirtualBox, Vmware or others for those operating systems.