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 Archambault knight aren’t necessarily the best games in their respective series.
1998 comes up a lot in response to this question, for good reason. Pokemon Red/Blue, Baldur’s Gate, Metal Gear Solid, Thief, Half-Life, Fallout 2, StarCraft, and on and on. Games were made much more quickly back then, and the technological advancements allowed for a lot of these games to do new things that no one had done before, that were quite predictably going to be well-received.
If I’m putting together a pantheon of great years in gaming, it looks like 1998, 2004, 2007, 2011, 2013, 2015, 2017, and 2023. If I’ve got to pick one, it might be 2004. Half-Life 2, Metroid Prime 2: Echoes (an odd choice for many, but it’s maybe my favorite in the series), Metal Gear Solid 3: Snake Eater and Metal Gear Solid: Twin Snakes, Halo 2, Burnout 3: Takedown, Star Wars: Battlefront, Grand Theft Auto: San Andreas, Unreal Tournament 2004, The Sims 2, Doom 3, The Chronicles of Riddick: Escape from Butcher Bay, Viewtiful Joe 2, Ninja Gaiden, Counter-Strike: Source, etc., etc. This was a magical time in online multiplayer, where it was pretty new for most, and you could do things like proximity chat in a shooter and expect people to actually use it for the video game at hand instead of spewing slurs into the mic. Local multiplayer was abundant. Obtuse game design made to sell strategy guides was just about obsolete, and DLC had yet to be invented (outside of beefier expansions). Midnight launches were exciting, and I have fond memories of, for reasons I can’t explain, playing Halo 2 on launch day in a 12-player LAN using bean bags, projectors, and 3 Xboxes set up in a local college’s racket ball court.