no set order for me
I base my decisions mostly on speed and features, and whether the investment is worth it over whichever model I'm replacing - going by phones which I purchased/upgraded to that were flagships at the time:
Got a 4 due to the Retina & FaceTime,
Got a 5 due to the lighter weight and the LTE
Got a 5S due to the touch ID and speed increase
Got a 6 for the battery, screen, faster wifi (but then sold the 6. Only iPhone I've ever sold that was a flagship model at the time. I really liked the design and curved edges...very comfortable to hold, but for the price was not a convincing upgrade overall)