IPhone devices feel more premium in your hand, ios is much smoother than Android, applications made for ios are better in terms of graphics, features, smoothness, etc.
Operating system gets more frequent updates, ios has matured and adopted more functions like widgets, control center, etc. IMessage and facetime are powerful features once user gets used to. Finally the system security, Long lasting device lifetime....etc.
Let me say that I have used all flagship devices made by Samsung and recently sold a S8 plus (too buggy compared to my 7plus) regardless of the ton load of features I gave up by abandoning the the android.