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.
1998 and 2004 have strong cases, as other comments have mentioned, but I think 2007 has got to be up there as well. The Orange Box alone was massively influential, even for just the new-to-'07 releases (TF2, Portal, HL2EP2), and was almost entirely unique - I don’t think we’ve really seen anything like it before or since. Beyond that, you have stuff like Halo 3, CoD 4, Assassin’s Creed, Super Mario Galaxy, Mass Effect, Uncharted, Pokémon Diamond/Pearl, and Guitar Hero 3.