It is easily gen 2 to gen 3. The following things were started in generation 3:
Music in games became the norm.
Graphics good enough that you could, for the first time, create lasting characters (Mario, Link, the DQ monsters, etc...).
Light gun games became available.
The D-pad replaced the joystick.
The most popular games were made for the home instead of being arcade ports.
Games could save your progress for the first time.
The first 20+ hour RPG type games.
The introduction of cheats like the Game Genie and Konami Code.
The creation of many popular franchises: Mario, Zelda, Metroid, Tetris, Dragon Quest, Final Fantasy, Castlevania, Metal Gear, Mega Man, etc....
No other generation boasts anywhere near this much innovation. In reality my first item, music becoming the norm, is a gigantic innovation just in itself. And yet that is one of many, many innovations all taking place in just one generation.