Its Maemo, and Maemo is pretty much dead, replaced by MeeGo. Also you left out Symbian, 2nd largest Smart phone OS in market. My favorite is Android, been using it for 1 and half years and quite happy with it. My current phone is SE Xperia 10 and next phone will probably be Xperia Play.
Android. And even if it's not the best right now, it will certanly be thanks to it's open source nature
Symbian is as good as dead... even Nokia decided to abandon it and go with Windows Phone... not that this was a good decision anyway
HELLO my favorite operating system for mobile is android it is noe to the market but it is rocking it has many functions and application and it is open source software.