S60, including offline.
(That's Symbian S60 on many, many Nokia (and more) phones. And someone above said Windows Mobile is bigger than iPhone. S60 is several times bigger again, I think something like 50% market share.)
Do you think there is any chance at all of an application for S60 (Symbian / Nokia)? It is by far the biggest mobile phone platform, and there are no GTD or To Do applications for it. Projekt is good but doesn't sync with anything off the phone.