Windows phone. I really liked Windows phone and how it meshed design wise with Windows 10 desktop. I also don't like the duopoly that iOS and Android share. I felt Windows 10 mobile was a pretty good OS and mainly it was the lack of apps that was just insurmountable and ultimately made Windows phone a non starter. However, I'm also disappointed in how Microsoft handled their mobile efforts, Nokia, carriers and developers. I still think they could have still been a solid 3rd mobile OS worldwide if they did not give up.