Hard to say.. WP initally looked like it could be promising, but I find it limiting in it's own way - if it was rootable, would probably be pretty good, but stock as it is, using it much can become clunky and cumbersome at times - takes so long to move around just to do a few simple things, and not customizable enough to add the shortcuts and such you'd like; but ofc being a W8 device, it leans more toward the new metro-y deployment, and less get-stuff-done usable windows that used to be great. Not to mention the comparatively slim offerings for app choices - part of that no doubt being lower developer motivation to cater to a smaller market.
iPhone is not bad, especially w/ jb to give you a decent amount of functionality and user control - not to mention actually being able to access filesystem to get stuff off w/o having to go through convoluted backup restore and whatnot (more a WP issue - esp w/ no root option); but there's always the apple-syndrome of paying exorbitantly more for what you get device-wise, on the grounds of I guess the aesthetics, or "cool" "in" and "hipness" that apple seems to always be trying to cloak its products in.
So it seems the best choice may have to be android; while I don't like google and their blatant disregard for personal user data, privacy, etc. that they exploit for their own benefit. I liked android better when it was a new open source idea and contender - a true alternate option brought about by the community - but I will admit it probably wouldn't have gotten to be as prevalent had it not been backed by a giant. As it's the system on so many different phones, it really gives the most flexibility, and is kind of the only choice if you want a device from most any alternate phone manufacturer. It seems to offer enough customization and user control as it comes stock, plus the root option really makes it the most versatile.
At any rate... well.. guess there's mozilla os...