I think Vita was increase in quality, even if it didn't find as much commercial success.

NES to SNES.
Huge improvement. Relatively few NES games I like honestly, but the SNES had plenty.

N64 to GameCube is another big one and if it wasn't for a few key N64 games (mainly OoT and MM) I probably would consider that the biggest leap

Wii U to Switch, NES to SNES, N64 to Gamecube.

Maybe, but if you were an arcade game fan, the Dreamcast was pure heaven.

Still is. Play them both on a reg basis

WiiU to Switch and Saturn to Dreamcast are pretty much the only real candidates; if looked at quality in the way of “this sucked” and “this was great”.

Master System to Genesis was a huge leap for Sega. Very few Master System games have stood the test of time, while the Genesis has a lot of classics.
And both SNES to N64 and Genesis to Saturn were huge leaps in my opinion, the move to 3D opened up so many options.
I don't get the Saturn bashing here, I guess some are just not very familiar with the console. It was amazing! It has a larger library of games than any other Sega console and three times as large as N64.
If anything the Dreamcast was a step down, even though it was an exceptional console in its own right.

Wii to Wii U I suppose.

PS3 to PS4 a close second.

NES to SNES. Good as the NES was for the time, its games aged very poorly overall, while SNES games are timeless.

Master System to Genesis. Similar to the above.

Game Boy/Color to GBA. Again similar to NES to SNES.

Wii U to Switch. The difference in library and functionality is gigantic.

For me, Gamecube to Wii. Going from a great but 'samey' system with a relatively small library and a decline in the quality of many first party titles due to rushing them out in an unfinished state to a system that changed the way games are played and whose innovations are still used today, and has a library that is both larger and better than its predecessor. Even some of the best Gamecube games like RE4 and Metroid Prime were much better when rereleased on Wii.

3DS to Switch. It's a huge frikken upgrade in every way. Obviously the graphics are a huge jump forward. But on top of that, I could never play a game that was as big and impressive as Breath of the Wild on a portable system before. It's a massive upgrade.