3GS: gift from sister, secondary phone until iPhone 5 became my primary
5: now secondary phone
6+: given to T-Mobile
6s+: current phone
4S: tertiary phone, passed to me when my son got my wife's iPhone 5
Never had the 3G or the 3GS and have never had any of the Plus versions. I have not kept each iPhone for the entire yearly cycle though and some only lasted a few months before being ditched for Android, Windows Phone or BlackBerry.
iPhone 4S, iPhone 5, iPhone 5S, iPhone 6, iPhone 6S, iPhone SE... No more iPhone after that. Conmpletely switched to Android on phone side. Currently rocking Huawei Mate 10 and P10 along with several other Android phone.