It's tough to pick just one.
SNES to N64: This is a pretty obvious one, mostly in changing design philosophies for games that weren't possible on SNES.
Fifth Generation consoles to Sixth Generation consoles: Saturn to Dreamcast, PS1 to PS2, N64 to GameCube.
PS2 to PS3: The best-looking games on PS2 aren't close to The Last of Us.
Wii to Wii U is a pretty big leap, maybe one of Nintendo's biggest in specs for home consoles. The issue is that the Wii U processor was weak, and the architecture wasn't inviting. Unlike Switch where several games (at least) push the specs to their limit, I'm struggling to think of a game other than Xenoblade Chronicles X and its open world that truly showed the limit of Wii U performance.
3DS (even New 3DS) to Switch is a big leap, whether in handheld mode or docked.
I'm not really going to compare previous gens to mid-gen of their successors because that seems obvious. Going from an Xbox 360 to Xbox One X would be one of the biggest spec leaps in gaming history. Even a PS3 to PS4 Pro would be quite huge.
Lifetime Sales Predictions
Switch: 156 million (was 73, then 96, then 113 million, then 125 million, then 144 million, then 151 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima
