Windows XP seems to have had a good run
(Although that's a visual impression, I haven't attempted to tabulate how long each one stayed either in or near the top of the list)
And of course, there's the old argument as to whether early varieties of Windows were operating systems or DOS apps
Not sure if it's valid to include mobile OS's in the same list as those for "proper" machines... but not sure if it would be right to leave them out either
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