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

[The_Liquid_Laser]

Here are the ones that stick out in my head (going from least notable to most notable):

5) PS1 to PS2 - I put this in 5th place, because there are a lot of great games on the PS1, but to me it is mostly an RPG system.  The PS1 shined when it had huge games that were slow like FF7.  Then the PS2 comes along and load times aren't an issue anymore.  The PS2 has great RPG's also, but it also has fun action games.  PS2 just has a more diverse library of fun titles compared to the PS1.

4) SMS to Genesis - SMS was a technically impressive system that was just getting steamrolled by the NES.  The NES was simply creating several of the most iconic game series of all time and it also got all of the third party games.  Then the Genesis comes along.  It has 3 Sonic games not even counting the CD-Rom or 32X, while the SNES only has 1 Mario game.  Genesis also got a very good third party library of games, while Sega was putting out some of the best games it ever made like the Shining Force and Streets of Rage games and also arcade ports like Golden Axe.  The Genesis is the reason why Sega is still thought of so fondly today by many people.

3) Wii U to Switch - The Wii U was a flop with a few fun games.  The Switch comes along and has the biggest game library ever.  Saying there is something for everybody on the Switch is an understatement.  Combining the home and handheld resources really did lead to a ton of games, although most of them are actually third party games.  And of course the fact that it can be played at home or portably is the icing on the cake.

2) Gamecube to Wii - Gamecube games are not fun.  It was just an inferior iteration of the N64, including failing even harder than the N64.  Then comes the Wii as the most innovative system ever made.  Wii games are pure fun, and they are like nothing else.  The Wii also had a library several times larger than the Gamecube, and it brought Nintendo back to the perception of being a serious developer in the home console space.

1) Gameboy to GBA - This assumes you count Gameboy to GBA as one generational step.  But so much happened from the original Gameboy to the GBA SP.  Consider that the original Gameboy was a black-and-white brick that ran on batteries and was not backlit.  Gradually all of that changed, first color, then some shoulder buttons, but the SP model really innovated with chargeable batteries, a backlit screen and a clamshell design.  On top of that it has this fantastic 2D library of games including many NES and SNES ports, as well as a huge library of original games.  There are even some PS1 style games like FF Tactics and 2D Castlevania.  Handheld gaming really came into it's own with the GBA SP model, and there was this great, diverse library of 2D games to go with it.  For me this has to be the winner of biggest improvement over its predecessor.