I’m thinking 2015 - Witcher 3, Undertale, and Kerbal Space Program are all classics. Fallout 4, Arkham Knight, and Cities Skylines were all excellent too, though fallout 4 and Arkham knight aren’t necessarily the best games in their respective series.
2001 is hard to beat:
Also Smash Bros Melee, Animal Crossing, Pikmin, and Luigi’s Mansion. Sonic Adventure 2: Battle too, but that was a port with added features. Gamecube lineup was STACKED that year.
Oh yeah, that’s definitely around the time when games started getting “big”, especially with Halo.