The iPhone 8 is my pick.
The 4 was an iconic shift from plastic chassis into a classy and premium feeling glass and metal chassis. Antennagate aside the device was undeniably premium to look at and to interact with. I remember how impressed I was when taking it out of the box and placing it next to my 3GS on the table.
The 5/5s kept the design and increased the size a little, introducing TouchID for the first time.
Then came the 6, this increased the size even more and added what was at the time GIANT with the 6 Plus. The feel in the hand with the rounded edges was really well received by myself and using touch ID made those round edges super appreciated as you grasp your fingers towards them as you reach for the sensor.
The 6s did not change the design.
The 7 changed the design slightly and there was an elusive "Jet Black" colour which was a scratch and fingerprint magnet but by the gods it looked sleek! Matte Black also was a great look, but I found the stripes running through the metal on the others to be a bit of an eyesore.
Then came the 8, it did for the 6 design what the 4 did for the 3GS, it gave it that premium flair and elevated the now 3 year old design to a new level of class with a beautiful glass rear. The camera bump was surrounded not by a soft metal slope but a much more stand out ring on the outside of the lens to contrast with the glass phone.
The iPhone 8 looks premium on both sides, got rid of the (imo) tacky white fronts and brought wireless charging and the A11 to a classic form factor. I am still using it today, it is still extremely fast and while everyone with FaceID is fumbling around with PIN codes to unlock their phones while wearing face masks I am still enjoying TouchID.