If we’re judging each phone as a product of its time and looking at the impact they had at time of release, then for me it’s the iPhone 4. Beautiful design that really stood apart from the competition and the first iPhone to really compete in terms of camera ability. The iPhone 4s was a strong refinement of this.
Judging them all by current standards, for me it’s the 8 Plus. The iPhone X was an interesting direction to take, whichever way you look at it, but it hasn’t exactly set the world on fire and there’s still a lot of consumers to be won over.