Android or iOS. Probably iOS - it's pretty "intuitive" and has all interesting for me apps (the only thing that matters in mobile world, especially after the novelty of fancy GUI modifications wears off). I've tried new BB phone - it was annoying to try and figure out the whole "swipe this way, then that way, then a little bit more swiping, then draw some doodle with your fingers to make some other menus to pop up" and the other mobile OSes already do everything I need...