Fortunately for me, i've used all devices, some longer than others.
I've been a windows phone user since focus s, 900, 920, 620, and 1020 and i recently just got an iphone 6 plus.
I've also had experience with an S3, S4, and S5 and briefly used a z10 which i thought was completely horrendous to use t the point i recommended my company to not buy them which we didnt.
IMO though, wp8.1 i wish was the original release of windows phone, if it was, i believe windows phone would be in a better position, but still third. Wp7 and Wp8 which i thought were both very similar were missing so many things and playing catchup to a very competiive market hurt the platform. With that said, after using 8.1 since the preview and experiencing the ios 8 now and s5, to me windows 8.1 does so many things better, faster, and easier than the other platforms. From just navigating your phone, changing settings, changing settings for specific apps, searching, the back button (which i miss) the notification bar which i think works great on wp, and just many other small things. Simply, 8.1 is the easiest of platforms to use amongst the three competitive ones, IMO.