It depends on what you're measuring. You can look at specs, software, markets, etc.
I say fourth to fifth. The shift from sprites to polygons was huge. Although fifth gen graphics are super dated, they were revolutionary. And let's talk about controllers. The N64 controller had an analog stick, and optional attachments (haptic feedback, memory, etc.). And as PlayStation controllers evolved, they lead to the DualShock. The DualShock is pretty much the first modern controller, and the blueprint for most controllers after. Two analog sticks, bumpers and triggers, haptic feedback, etc. CDs were the main game media for this gen. Obviously, Nintendo and Atari (but Jaguar barely made a splash at all) were holdouts still.
You also had multimedia features like music CD playback. I know CD-i could play music CDs, but it wasn't mainstream.
Lifetime Sales Predictions
Switch: 151 million (was 73, then 96, then 113 million, then 125 million, then 144 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 67 million (was 60 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