The android OS was also very good because it's mostly Open-Sourced and enables users to customize applications as much as they want although this also means that the android OS is less secure as I also learnt that 1 in 5 of the apps has secruity holes.
The Blackberry OS had some useful features a few years ago but now those features have pretty much been perfected by apple. One of strong features of the Blackberry OS was the instant messaging service which was a very popular service at one point.
The Windows Mobile OS is an emerging OS which has not yet become popular but does implement many of the features of the other OS's
The Symbian OS is very popular amongst cheaper phones and can be found on all Nokia phones. The Symbian OS has very limited features and would likely only include features such as Calender. Research have shown that most people dont know Symbian is even on their phone.