Obviously that is subjective. But for me it's all about integration of my mobile devices.
Windows could be just as good with a Windows Phone (I honestly don't know) but I'm not interested in a Windows Phone. Android and Windows, while they work perfectly fine together don't offer the experience or level of polish I'm looking for.
I use handoff, continuity, message app, iCloud Photo Library, Pages, Airplay, iMovie, and a ton of other things between OS X and iOS so regularly now I'm nearly lost without them. For example, transferring photos to my computer seems like a chore now that I don't have to anymore or quickly responding to an SMS while I'm working on my computer or answering a phone call.
So for ME OS X is far superior. If it wasn't for those reason I could care less. When I first came to OS X I just loved how I didn't need to tinker for days to get things I felt should be trivial to work in spite of its obvious limitations which I'll offer no argument that they exist.
Also in lieu of being a tinfoil hat wearer I trust Apple with my data slightly more than Microsoft....only slightly but enough worth mentioning.