What system was the biggest improvement over its predecessor (in overall quality, not graphics) and why?

[Chris Hu]

Even though it's the same system I say XBox One to XBox One S. The One S got a slight performance boost no longer has an external power brick and the disc drive got upgraded from a Blu Ray drive to an Ultra HD drive. Plus, it's smaller than the X1 even though they put the power supply inside the console.