8 bit to 16 bit. There was a massive technological leap there that allowed for very crisp and beautiful visuals. Although 16 to 32 bit was very close - some noted that PSX had poor 3D modelling, but it much more than made up for that with pre-rendering, redbook audio, and FMV.
After that, the gap between generations slowed down significantly.
Anyway, I am not a fan of unifying a bunch of different consoles together as one generation as Nintendo and Sony/Microsoft are on different tracks, and only aligned during the PS2 generation when Nintendo went with a PS2 clonebox.
8-bit to 16-bit is 3rd to 4th, 16-bit to PS/N64/Saturn is 4th to 5th.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023. (And over 130 million lifetime)