For me, that's iOS. This year Apple's releases in both hardware and the OS made that obvious to me. While I didn't set out to do so, I have totally dropped Android from my tool kit. Not because it's inferior in any way, simply because it doesn't work like I do.
Much more so than most who have written about why they moved from Android to iOS (or vice versa), Kendrick lays out a fair look at what it was that made him change sides in THE SMARTPHONE WARS. Ahem... I got a little carried away there.
We have multiple platforms that are very good at this point. I similarly made the move back to iOS this year, and I made the change because iOS made more sense with my workflows and with my lifestyle. I know a lot of people who swear by Android and say the exact same thing about it. Neither side is wrong, and we should be trying to understand why people use the phone they have chosen, not try to break them down and tell them why they chose poorly.