Android is the most famous and mostly used Operating Sysytem worldwide. Appx. Evry 2nd smartphone in the world is an android powered.
Due to the open source nature of android it is the most famous and experimented os.
Custom ROMs are developed and skinns are develpped to provide hell lot of customisation, only bcoz of the AOSP project launched by google.
This os is not buggy, and juice out the overall performance which a device can give.
In my opinion this is the best OS in the world for any smartphone.