Fourth Generation = Beating the Pokemon League with only Mewtwo and no items in Pokemon Yellow. (Closely followed by Sonic The Hedgehog on the Megadrive)
Fifth Generation = Beating Gundam Wing Battle Assault 2 on the hardest difficulty as every character.
Sixth Generation = Playing Rogue Squadron 2: Rogue Leader for the first time. That game was a graphical/audio powerhouse.
Seventh Generation = Playing Wii Sports for the first time. Then I played it again and again and it never got old.
Eighth Generation = The Legend of Zelda: Breath of The Wild. I stumbled across a possesed dragon atop a mountain and had to fight it but I only had two arrows, two blunt swords and the battle was in the air. I fell down cliffs, into trees, into nearby enemies, i got struck by lighting. So much was against me but I won in the end and I'll never forget it.