Very hard to decide. The original iPhone obviously had the biggest leap in innovation as it permanently changed the smartphone industry. The 4 had the biggest leap in design (IMO) and that retina screen was just unheard of in a phone before. 4S brought voice assistants to the mainstream. The 5 was strangely the iPhone that got me to switch from Android, even though I didn’t think there was anything remarkable about it. 5S revolutionized how fingerprint scanners were implanted in smartphones, and of course started the 64 bit trend. The 6 was awesome because it was so strange/cool to see an actual large iPhone. 6S was a powerhouse that still beats some 2017 flagship phones if I’m not mistaken. The 8 has what may be my new favorite iPhone design, and the X is the first iPhone to truly introduce a new front design. Like I said, hard for me to decide!