3rd Gen - Super Mario Bros (1) or Super Mario Bros. 3, both insanely large impact on the gaming world. (honorable mention: Legend of Zelda)
4th Gen- Super Mario World or Donkey Kong Country- both best selling games of that generation and were revolutionary for the time and spurred a lot of copy-cats. SMW in particular was somewhat revolutionary with its mix of platforming and overworld map selector
5th Gen - this is a tough one. Torn between Ocarina of Time and Final Fantasy Vii. Both were hugely significant for the adventure and RPG genres. (Honorable mention: 007 Goldeneye as it popularized the FPS genre which would become debatably the biggest genre in video games, sorry but the likes of Doom and Wolfenstein at that point had not been massively popular)
6th Gen- Metroid Prime or Halo. I consider them the pinnacle of that generation (honorable mention: SHadow of the Colossus)
7th Gen- maybe the Last of Us? just in terms of acclaim and significant use of newer technology to really immerse the user in a story and gaming experience
