The iPhone 4 would be IMO, the first feature complete iPhone bringing multi-tasking and home screen wallpapers.
iOS 2 brought us the App Store (correct me if I am wrong)
iOS 3 brought us Copy and Paste, MMS
iOS 4 brought us home screen wallpapers, FaceTime (iP4), multi-tasking, and useless features like Game Center.
iOS 5 brought us a new Music App, and most importantly, Notification Center
iOS 6 brought us random features like color matched status bars in Apps, as well as new Apple Maps, Passbook, etc.
iOS 7 brought us the redesign, and a lot of new features like Air Drop, Control Center
Of course theres more, but I just got off from work, and am really tired. Waiting for my gf to get home.