If you're talking about the mobile version of Windows, then Android reigns supreme. The numbers speak for themselves. There are way more Android devices out there than Windows phones and for good reason. There are way more apps, you can do more and it's integration with Google's services, e.g Gmail make it the ideal choice for many people. Apart from Microsoft's Lumia 950 and Lumia 950 XL, there are very few great Windows-running phones in terms of hardware compared to the vast amount of options when it comes to Android devices. There are a few budget offerings but Android running devices dominate the smartphone market, after iOS. Just to put things into perspective, iPhones accounted for 91% of smartphone sales in 2015. Windows phones - less than 1%.
But of course it comes down to your personal preferences and whether you want something as advanced and in depth as Android, or something as simple and straightforward as Windows.