Currently its a two horse race, and from the looks of things, Android will stay on top for a long while. The three biggest smartphone OS currently are Android, iOS and Windows, with Windows targeting the iOS users, not Android, in terms of features and functionality. In ecosystem design its closer to iOS than Android, but brings a bit more freedom in what it allows and what it does not. It will still take time before it catches up to the two, if it ever will, but based on that, I don't see it dethroning Android.
Tizen, Sailfish, Bada and other smartphone OS are just either too new, or has too little ecosytem and financial support to become any real threat to iOS and Android. Even Windows has a big "?" hanging over it.