Maemo: Great platform but has a proprietary user interface and other component are closed. Limited to Nokia hardware and some obscure devices.
Android: More open than the rest but google apps are closed and source code can be withheld (in the case of honeycomb). Widely available. Not the best architecture.
BBX: Great Unix like proprietary platform with some open components. Walled garden. Limited to Blackberry hardware.
iOS: Some open components but a locked down proprietary platform that is a walled garden. Limited to Apple hardware
Firefox: Truly open and not a walled garden. Some available hardware. Limited to web apps.
Tizen: Open source and not a walled garden. Uncertain future.
Sailfish: Open Source and not a walled garden. Best of them all. Limited available hardware.
Windows Phone: Proprietary and walled garden. Some hardware is available but all the devices look the essentially the same. Based on that Windows crap.
Ubuntu Touch: Incomplete and has licensing issues. Nice looking user interface. Runs on existing hardware and will have hardware of its own. More available than previous mobile linux efforts. Open source and more open than previous efforts. Not a walled garden.