I myself used : Windows Mobile, Android, Apple iOS, Chinese MTK, Symbian, Blackberry, & Linux.
Windows Mobile: Buggy mainly due to ignorant operators & microsoft closed approach.
Android: Future of mobile OS, still under development & going great.
Apple iOS: Most stable OS with flexibility of millions apps but best come out when jailbreaked.
Chinese MTK: Provided power of mobile communication to poor in developing countries, imagine mobiles starting from less than 2$ without any contract.
Symbian: Most widely used OS but now in dying stage.
Blackberry: Only useful due to blackberry e-mail services otherwise not usable.
Linux: Mainly for testing purpose.