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.
I’d go back to maybe 1997 bit there are so many good years to choose from.
This is the answer with '98 a close second