I have an iPhone 4S and it has done its job well enough for the longest time, plus it was free so that's a bonus.
I am however wanting to replace it now and likely going to get a Sony Xperia Z2 or Z3. I really really dislike the Galaxy S5, because the polycarbonate body on it is in my mind cheap. I don't mind and in fact prefer the phone to have some heft to it, so the metal/glass body of the Z2 is right up my alley. Plus it's got the same processor but 50% more RAM.
Supposedly the HTC One M8 is also brilliant. The Nexus 5 isn't bad. In my opinion right now it's:
Xperia Z2 = HTC One M8 > Galaxy S5 > Nexus 5
Forgot the G3 too, probably put it right in above the S5 as well.