I started out on hard. Regretted it, but I got through it with enough asshole puckers.

To me, I think it's Persona 5 Royal, which should be the blueprint jrpg going forward.

For open worlds, San Andreas revolutionized how worlds work, but that can also be traced to OoT.

Fighting games, I dunno, Smash maybe? Some of the earlier Streetfighters?

For saving the gaming industry, Super Mario Bros 1?

What I'm getting at is this is too broad of a question. Every type of game has a good contender and it is based largely on the type of games you play and like. Fps's have Goldeneye and Doom which concreted the type of play even to modern CoD games. There is no easy answer here.