Windows Phone.
I felt like the OS hit a stride around 8.1, with some markets sporting somewhat impressive marketshare, and the corporate politics of the whole situation and Nokia merger screwed it up badly.
I really think if Microsoft had doubled down and focused on getting flagship devices to all 4 flagship carriers it would have gone somewhere.
But I remember at the time having a dead end of hardware where competitors were putting out new phones on all 4 carriers every year. With Windows Phone you were hopping between carrier exclusives or getting nothing because all the new Nokia/Microsoft phones were low end or mid-range at best.