Apple iPhone with iOS, some Androids with many versions showing the fragmentation issue.
And for legacy some Windows Mobile 6.x version.
All work fine so I'm going to answer I use them all but have no favorite.
Oh wait, I forgot that I have one last Palm Treo with PalmOS. Still works!
Now in the market Android is open source and having huge market for it. And now Cyanogenmod , Firefox, and other also coming. Is cyanogenmod is good or Android.

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