Well, I prefer Symbian just because its easy to use as well as there are several softwares available for it. Right now I am using Symbian S60 V3. - Mr.President -
iPhone here - simple, lots of some very good apps (many, many rubbish ones). The phone is quick as well (3GS).
iPhone mostly since it's what I'm using at the moment. To me, the iPhone OS is super easy to use and works very well!
I'm using at most WM, but the iPhone OS looks better than WM. For mobile phones I prefer Symbian. It is so hard to chose the perfect one.