4-5: 3D and FMV explosion, it was something to behold.
2-3: Super Mario Bros. changed the landscape of videogaming. Still, 3rd gen's early years were still dominated by arcade games in the style of 2nd gen. Consoles like the SG-1000 also overlap generations.
1-2: Underrated answer. 2nd gen standardized actual game libraries, greatly expanded on genres when the 1st gen only had like 3 of them in very raw formats (sports which were Pong variations, light gun, racing), level progression even if most loops were short, in-game backgrounds, high scores. You have the progression of dots and sticks to Coleco and SG-1000 graphics that look like early 3rd gen.