I've been through an iPhone 3, 4S, and now a 5S (now several years old) and have not had a problem with any of them other than my dropping them causing display cracks. I try to keep my battery at near the (ideal) ~40% charge level never letting it get below 20% or charging above 80% and batt life is still near new levels.
I can count on one hand the number of crashes I experienced, however, I am apparently not the typical user as I don't install a zillion apps for everything (no facebook, bank, games or other crappy apps - the stock apps have been stable).
I chose the iPhone 5S because I like the chunkier, meatier form factor and the 6s seemed so thin I am afraid I'd break it in half if I sat on it in a back pocket. I believe the thinness of the 6 is what causes alot of trouble due to the entire frame flexing.