I’ve owned the 3G, 3GS, 4S, 5, 6S, SE (original), XR and now the 13 Pro. It’s too soon to judge the 13 Pro, I like everything about it so far especially the camera, but we’ll have to see how it holds up over the long term.
I have two candidates for favourite - the first having to be the 3G. As someone who had been playing with earlier smartphones for years before the iPhone (and who dismissed the original iPhone as it didn’t support third party software) the 3G along with the introduction of iPhone OS 2 and the AppStore really impressed me. It’s easy to forget now just how far ahead of everything else on the market the iPhone was at that point, in terms of speed, ease of use, and the sheer quality and quantity of apps and whole new capabilities that started spilling out of that store in the early days. It was a fun time and a really enjoyable, pioneering device to own.
Second is much more recent - the XR. After eleven years of using iPhones, it was the first one I owned which did away with the home button, had that much bigger display and brought in Face ID, refreshing and improving the basic experience of interacting with the phone for the first time since the 3G. It was also the first iphone I owned which really had killer battery life, to the point where I simply never had to worry about it making through the day, or probably the next, throughout the whole two+ years I used it. For various reasons it made sense for me to upgrade it, but I enjoyed using it right to the end and it would be no hardship to go back to it now, which isn’t always how I‘ve felt about iPhones I’ve replaced in the past.