iPhone 4.
It genuinely was a generational leap not just in performance, but appearance as well.
The phone went from plastic materials used in the prior models to stainless steel, screen got upgraded to the Retina display along with increase pixel density, camera specs was upgraded to 5 megapixel along with LED flash, second microphone for noise cancellation, gyroscope, front facing camera long with the introduction of FaceTime.
It was a beautiful premium phone, & Steve Jobs presentation of it was quite fun to watch.
It would've been better than it is today. Also IOS, would be nowhere near as buggy.