I'll never bow down to any manufacturer that wants to lock me into their "ecosystem" or whatever they want to call it as long as there is an alternative that lets me do what I want without voiding the warranty or becoming a criminal (seriously look what happened to the poor guy putting linux on his ps3).
Maemo and MeeGo seem to be the most open OS for a lot of reasons. Nokia does a pretty good job of producing the hardware and giving a few updates, but mostly leaving us the hell alone. I don't need to buy their charger, their headphones, install their software, use their email, use their themes, use their app store, transcode media into their formats, unlock or hack anything to run unsigned code, or pay them anything extra to develop software (unless you want a commercial Qt licence). It's really my device to do what I want with. If I want apps, they end up mostly being free since open source phones attract open source developers. Open source code is usually safe, so that's a plus too. There's also a lot of settings we can adjust to change performance characteristics or customize the appearance that you just can't do on other phones without a lot of work.
Anyways, I am supposed to be writing some open-source code for the MeeGo competition, need to get back to work