This year: The Legend of Zelda: Breath of the Wild. Goddamn what a beautiful way to introduce a new console.
Recent times: Fallout 4. I've never been so immersed in a game in my life. Everything that steered me away from Fallout 3 and New Vegas (both of which I tried to love, having loved Elder Scrolls: Oblivion* and Skyrim*) was gone, and most of the new stuff was to my taste - the weapon and armour customisation and particularly settlement construction were both features that I loved, the latter sucking up probably 100 hours of my playtime overall.
All time: Bully. Of all of the games I've ever played, Bully has been the closest to perfection. Everything - the map size, the missions, the side missions, the things to do outside of those - has a perfect quantity, never outstaying its welcome and always being enjoyable. The game is like Grand Theft Auto with less malice, less death, and honed and refined so there's no excessively large areas on the map, no single difficulty-spike missions, no overlong side quests - even the weakest bits of Bully required to hit 100% (the arcade games and the shooting range in the carnival) are done quickly enough that they're not too problematic.
Other notable games - Minecraft would likely be my most-played game, Pokémon Yellow would be the game I have most nostalgia for that I can no longer enjoy very much (too slow, too buggy, mer), Super Mario Odyssey is my most anticipated, and Spyro the Dragon 1-3 on PS1 would be most influential - they're the reason I took such a vivid interest in the idea of making games.
*to provide a counterpoint to the person above stating Morrowind as their favourite game, I actually really disliked Morrowind, where Oblivion finally sold me on Elder Scrolls. While it was a shame that some of Morrowind's more outrageous possibilities were toned down or removed in Oblivion, Morrowind was way too much of a stat-juggler for me. Luck especially did my nut in - when I visibly watch as my sword swings through an enemy, I expect that enemy to take damage - not have some random dice roll, in a real-time RPG, tell me that I didn't hit! It's different strokes for different folks for sure, I just prefer my games with fewer things to worry about.
And I would welcome if Elder Scrolls 6 came with an option to completely 100% remove any ability to permanently balls up your progress - I still haven't got over the fact that during my first run in Skyrim, I accidentally hit a chicken in Riverwood right before the game decided to do its first autosave. The entire village went aggressive on me, including the man needed to continue playing the game. I had to redo the entire first hour of play, including the INTRODUCTION THAT TAKES A MILLION YEARS, so I could actually play the storyline. But only an option, if you're masochistic enough to like that you can obliterate an hour's gameplay by nudging a PS3's nasty squidgy R2 trigger and trigger a sword swing on Mrs Hen, Love of the Village, then you absolutely keep playing like that!