The Galaxy S5 was a good phone, but nothing stood out with it.
When the S4 arrived t brought a Full HD Display and a plethora of sensors and Features some of which other OEMs are only implementing now and Samsung has strangely removed like the Gesture Sensor and IR Blaster.
The S5 was more like just a gap filler while Samsung worked on their new design language and hardware (aluminium build, UFS Memory, new UI etc...).
The S6 was the 1st gen of the Samsung we know now, and 1st gen is almost always never perfect, and has many flaws.
S7 was the 2nd gen which fixed everything and the S8 is the evolution of the 2nd gen.
You can't go wrong if you get a S7 or S8, in fact, I believe the S7, S7 Edge, S8 and S8+ can coexist and do coexist well as each fill a nice little gap.
I wouldn't leave my S7 Edge for a S8+ but at the same time I wouldn't blame someone who is a tech addict for doing so as the S8+ is incredibly beautiful and is an evolved S7 Edge.