Debian Linux. Because it just works.
Runner-up: Mac OS. Same reason as above, but not free, so it’s #2.
Second-runner up: Free DOS because why not?
Distant last place: Windows, cause occasionally you need to call in your retarded cousin who is the only one that can do that one thing just right.
So when's he going to gather them all at Mar-a-logo of New Guinea and have them drink Flavoraid?