In terms of "how impactful is this model compared to what it's replacing", I would say the best update was the iPhone 6 or the iPhone X. Both fundamentally changed the entire iPhone (in terms of design), and added much needed technology, while also introducing new functionality to the world too.
The worst update was probably the iPhone XS/XS Max, because compared to the X, it changed barely anything. Slightly better camera sensor and the A12 (which wasn't that big of a jump from the A11 compared to other upgrades), and while it gained Gigabit LTE and dual-sim support, it also got stuck with a terrible Intel modem.
Honorable mention for BEST: iPhone 12
Honorable mention for WORST: iPhone 4s
although 6 and 6 plus were a plus in terms of design since they weren’t a 4 inch phone, you can’t justify and say it is the best, no way.
A8 is a minor upgrade over A7
Both 5S and 6/6+ got 1GB DDR3 RAM ( and 5s is a faster phone due to its screen size and less than 1 million pixels compared to the 6 plus), piss weak GPU (since it is only a minor upgrade over the 5s and especially for the 6 plus it was bad)
NFC and Apple Pay is probably the only good things “technological wise” over the 5s but even technological wise compared to the 5s it wasn’t even major so yeah I fail to see that the 6/6+ added much needed technology when apart from NFC and Apple Pay it’s improvements are ****
now to the X/XS, to a certain extent I agree but I disagree with you in saying the A12 wasn’t a big jump compared to the A11
5 trillion operations per second compared to 60
Third party’s can use the A12’s ANE compared to A11’s ANE in which it was only used by Apple (so largely it’s 600 billion operations per second isn’t really of much use for apps
A12 (and only the XS/XSM) can support 4GB of RAM compared to 3GB
A12 was the first non X chip to support some games at up to 60fps (A11 theoretically can but it had flaws which prevented that from happening)
plus not related but only 3 devices have A11.. we only saw new Apple TV with the A12 so you may want to hold it before saying A12 isn’t a big upgrade over A11. Maybe it’s clock-speed but Geekbench scores are still good for A12 compared to A11
also A12 during iOS 12/13/14 got features exclusive for it and higher that A11 didn’t get like QuickTake, Location Anchors etc etc etc