Apple's consistent unscrupulous practices (that is another hub by it self) put me off using their OS, and I will try not to let that influence my answer.
iPhone iOS; Fantastic for the "computer illiterate". Many more apps to choose from for now. Good solid OS which rarely crashes.
Android:Easy to use. Does not restrict uses when coping files from one device to another. Android Apps are generally free of charges. Android is a type of "for the people by the people" software. Also has rare crashes.
Windows: Best "universal" OS as it can be used many different types of devices such as phones, PC's tablets etc. Limited apps for now and apps cost more than Android.
Blackberry: Sorry there are no pros
Apple iOS: Most apps are too expensive. OS does not communicate easily with PCs. Strong problems when transferring files such as music and video regarding licences to own those files. Strong requirement to have an iTunes account. (who are these guys, they sound like some kind of communist government - Sorry).
Android: Not as many apps as Apple. More user training required if you are new to Android.
Windows: Still has bugs for the mobile sector. Limited expensive apps. Licence to own the OS is not free.
Blackberry: OS has a history of being cumbersome and difficult to use in recent months. System will crash frequently. Does not have as many apps as their competition.