Isn't every S upgrade (if not every upgrade) essentially the best model yet (until the next one comes out)?
I think the spirit of the question is asking to take all S-cycle upgrades, and in terms of the performance improvements relative to the prior model, rank them against one another. So was 4–>4S a better or worse leap/value/transition than 5–>5S, etc., and where do people think 6–>6S ranks among those?
I think 5s also represented a sweet spot because I expect it to age more gracefully than any of its predecessors, not just for internals but because of the 5-series screen size, which some people still prefer to the new sizes, and also because its hardware has been demonstrated to be built to survive a little better than previous models (and perhaps the 6/6+). None of the technology in it is likely to be outright abandoned soon (Lightning, retina, LTE bands, wifi and BT protocols), so it could last as the budget entry for a very long time. And, of course, it was better than the 5.
The 6s has the ability to "solve" some of the "problems" (such as bending and antenna lines on the back—two issues that may have been fundamentally related) that are probably not proportional to the amount of media coverage it received but are weaknesses nevertheless. Furthermore, a small but substantial increase in memory (1 GB–>2GB) would have huge implications for longevity [insert snarky reference to indulgent app developers here], and starting the entry level ($649) model at 32 GB would instantly depreciate the 16 GB iPhone 6.
