That most certainly depends on which kind of device I'm using!
I am, and have always been, a true Windows fanatic. Started out with W98, and from there I've been hooked.
When I got more involved with developing, I soon realized I needed a server, and from there on I've always had Linux servers with Ubuntu. I'm taking an education at the moment as a data technician, and they're trying to push down Windows Server on us, but even though the "true" windows environments seems to also use windows server 2016, I still prefer to have a simple ubuntu distro at hand, to run my various Discord bots and what not.
If we're including mobile phones, I'm and Android guy. Preveously an iFaggot (back when iOS was literally the only OS that ran smoothly on phones) but after Android has stepped up their game, there's no way I'm going back. My television also runs Android OS, which just makes it even more clear that I'm stuck at the droid.