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

[Veknoid_Outcast]

For me, it's WiiU -> Switch.

I have a soft spot for the WiiU, but let's face it: it was a disappointment. It allowed for some very cool asynchronous gameplay and hosted 6 or 7 of the best games of all time, but it was slow and clunky, it wasn't properly supported by third-party publishers (AAA or otherwise) or by Nintendo, and its signature controller, the GamePad, quickly became an albatross.

The Switch, meanwhile, is arguably the best video game system ever made. Incredibly versatile and accessible hardware, extraordinary support from Nintendo (that has lasted 9+ years), and plenty of brilliant "AA" and "A" games.

Some other contenders: NES -> SNES, SMS -> Genesis, GBC -> GBA