What system do you think represents the biggest improvement over its predecessor, and please explain why?

(And by quality I mean overall quality in your opinion, like if you personally would rank one system a 4/10 but it's successor a 9/10)

Wii U to Switch. Do I need to explain? The must better 3rd party support and vast library and the oh so sweet RPGs,mecha and action games. Shmups. Arcade ports. So so so so much more.

Xbox to Xbox 360 for the Japanese support esp. Again RPGs. Shmups (many exclusive at the time) and just a general broader appeal in general. Even if not into Halo or Gears. 360 had many times more to offer everyone than OG Xbox.

Dreamcast over Saturn was pretty nice.

Really though, I think the trend lines have been pretty consistent in their slope. With the possible exception of the WiiU, pretty much every major console has represented a small step up at the beginning of the generation, but ends up far better than it's predecessor by the end of the generation.

Quality is so vague lol.

I'd say PS2 to PS3 for graphics.
GameCube to Wii for innovation.
Xbox purchasing games to GamePass for money saving to consumers.

True, but kind of intentionally so as I mean quality as a matter of overall opinion. I should clarify that in the OP.

I'd say Wii to Wii U

Sure the Wii U was a commercial failure, but the games are good enough to be all ported over to the Switch. While the Wii has a few stand out titles, most of them are just shovelware not worth looking into. Also Nintendo's first jump to HD

Easy, Saturn to Dreamcast. Only thing close imho is WiiU to Switch.

Nintendo

• Hardware capacity
  it was from the Snes to the N64( hardware capacity wise ).from nes to snes was more of a linear progression, there was 3D on the snes but was more of a gimmick than anything else and games that use it have badly aged on the other Super Mario 64 as a launch title was truly amazing and still holds up today. After the 64 it was more of a linear progression.
• Support
  it's from the WiiU to the Switch as it enjoy better third party supports.

Microsoft

• Hardware capacity
  Linear progression all along. But from One to Series X will best all progression shown in-between previous gens.
• Support
  I'd say from Xbox to Xbox 360 for the quality of support shown during the first half of it's Lifecycle. I'm hoping the Xbox Series X|S overtake that and it appears to be on a good start (Xbox 360 early life on Steroids).

Sony

• Hardware capacity
  hard to say, it felt more like a linear progression all along. Maybe from ps2 to ps3 since spec wise it went from the least able system of gen 6 to the most during gen 7.
• Support
  I would say from ps3 to ps4 since the ps3 had a somewhat poor support on the first half of it's life compared to standards shown with all others.

Eh if you played the Japanese library of Saturn it was massive and amazing. I love the DC to death but it never had as many good shmups/RPGs/Fighting games as Saturn.

SNES to N64 and PS1 to PS2.

Completely changed the types of gameplay that were feasible on the systems.