Operating systems in popular usage which are still being maintained and support the features and hardware you would expect a current OS to support (SMP, Multiuser, memory management, to some extent hardware virtualization, up-to-date hardware standards, etc.) Primarily Windows, Linux, OS X and iOS, Android, etc.
In short, OSes which people are coming to, not those which people are running away from.
Or look at it at a consumer's point of view, any OS that supports the apps they want to run on consumer grade (and priced) systems, be it a smartphone, tablet, notebook, or desktop. If you can't run on all those items, your future is limited, very limited.
