It's got to be 1991 to me. So many icons and showing my age here.
The Legend of Zelda: A Link To The Past
Sonic The Hedgehog
Final Fantasy IV (II for NA)
Super Castlevania IV
Street Fighter II
Metroid II
Fatal Fury
Mega Man 4
Adventure Island II
Teenage Mutant Ninja Turtles: Turtles In Time
Lemmings
Battletoads
Bonk's Revenge
King of Monsters
Ninja Gaiden 3
Castlevania Dracula X: Rondo of Blood
1991's IP's were insane and straight up built companies that are still around 30 years later. Sonic got movies with A-lister casts 30 years later.
Gameplay, and modern games I think you have to grade separately.
However, the intro of characters like Guile, Dhalsim, etc. back in 1991 introduced the main fighting game metas/strats and basically who many fighting game characters got their whole style from 30 years later.
Dang near every fighting game you played and characters you enjoyed (even from other games) can be traced back to 1991: grapplers, zoners, rushdown, charge characters, mix-up, etc.
Even musically, all those sound effects from big ass top-charting songs are clips from these games.
1990-1992 was basically phenomenal. So few years have contributed to gaming as much.