I'm gonna go with 6th to 7th personally.
Not only was the leap from PS2/Xbox/Gamecube graphics to the kind of visuals we ended up seeing in the likes of the Uncharted and Gears of War games absolutely gargantuan, plus the greater scale and scene complexity, but the Wii's motion controls were a quantum leap for me in terms of gameplay. Being able to aim with pinpoint precision in Metroid Prime 3 or swing a sword with my own arm in Red Steel 2/Skyward Sword/Wii Sports Resort felt like something from the future brought back in time to the present.
Every gap before 6th to 7th was enormous as well.
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)