The Idol 4s isn't even close to the best Windows Phone of all time. For me, the best Windows Phone of all time would have to be a true flagship where you didn't have to make any disappointing compromises. The only Windows Phone that fits this criteria is the Lumia 1520. It was the first and only Windows Phone where you got the best of the current-gen tech without having to make a compromise. Even with the Idol 4s, you are making compromises on battery life, camera quality, wireless charging, and lack of NFC, just to name the obvious ones that spring to mind.
The only complaint anybody ever had about the Lumia 1520 is that it was too large for them. And that's just a personal preference, not a shortcoming with the phone itself. You could say that the 16GB AT&T version vs 32GB other versions was a compromise, but since you could get a 32 if you really wanted one, I don't hold this against the phone.
The Idol 4s is a decent midrange phone (when it actually works), and the X3 would be the obvious champ if its camera wasn't so terrible, but if we're talking about the best Windows Phone ever, it's hard to argue against the Lumia 1520. It is still holding up well years later. In fact, I've given up a 950XL and an Idol 4s to go back to my Lumia 1520.