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 was going to go with '06, but it looks like '04 was revolutionary for many game genres, and considered a major milestone in video game history due to its lasting affect on future titles.
https://en.wikipedia.org/wiki/2004_in_video_games