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

[Louie]

Wii U to Switch comes to mind. The Wii U was such a slow and sluggish-feeling system, long loading times, etc. The Switch felt way more modern in comparison.

Wii was also huge improvement compared to Gamecube. System felt so sleek with its new controller and design.

PSP compared to any handhelds before and also the original DS model. The PSP was an extremely modern, state-of-the-art multimedia system with this huge screen and incredibly slick design.