Design wise for me it's a toss up between the 4 4s and the 5 5s. The 4 when it came out was pretty amazing, glass back stainless steel sides, after the plastic 3g and 3gs the build quality seemed in a different league, and of course the retina screen, overnight it made every other screen resolution look like the 1980s. The 5 5s and of course the SE felt like a sleek refinement of the 4, it was lighter and had that sexy aluminium back and the extra screen size did make a difference from the 4 in use, my white iPhone 5 was my favorite phone so far.
My 6+ is a great phone but it's a tool and I don't enjoy the look, weight and size of it, but they are worthwhile trade offs for the bigger screen most of the time.