All of the iPhone are great these days. Anyone with an iPhone 6 or older should consider upgrading. The newest iOS 11 will likely cause havoc with the older 5 series. While the iPhone 6 is actually a good model, T-Mobile users would be better off with the 6s or later as the regular iPhone 6 does not support TM's 700MHz band. The iPhone SE, if you can deal with the smaller screen, is essentially the same guts as the 6S, but in the same body of the 4 and 5 series. My feeling is there has never been a better time to buy an iPhone 7 series model as they are deeply discounted.
Here is a small chart comparing the 8 to the X...
To see all iPhones and compare them.
Joel Saltzman has over twenty years of wireless industry experience. He is currently CEO and Chief Wireless Analyst for Dr Wireless.