As my username indicates, I like S models for their refinement over the original versions. My favorite by a good margin is the 5s.
The 4s was my first iPhone, and while I loved it at the time, the 5s just felt sleek and fast and much better. I loved that phone. After two wonderful years I traded it in at Verizon for something like a measly $130 (huge mistake) to get my current 6s, and 2.5 years later this size is finally starting to grow on me. I hope a smaller X or SE2 comes along this year, because I’m definitely upgrading to something.