Hard to pick just one, I can narrow things down a little though:
Persona 5
The Witcher 3
Fallout: New Vegas
Kingdom Come: Deliverance
I love the Persona series, the fifth game is 95% of the reason I still own a Playstation. It improved upon every aspect of the prior games and tied it all together into over 100 hours worth of playtime for your first playthrough, you're talking hundreds of hours in total to see and do everything that there is to offer. The style, music, gameplay, and story all make it the pinnacle of the JRPG as far as I'm concerned.
The Witcher 3 delivered in all the ways that matter, I was finally playing an open world game that felt like a real living and breathing world, one which was stunning without my having to spend countless hours of my life modding and tweaking. Something I never got from the likes of Skyrim, and better yet it offered an interesting story and plenty of subplots to keep things moving along.
Fallout: New Vegas is to my mind the ultimate Fallout game post the original series, flawed in many ways but a masterpiece despite those flaws. The world was a little bland, but the wealth of things to do and storylines to follow, the different ways you could build your character that had an actual impact on the world. The interactions with the various factions and how they effected the world at large. Even looking at the DLC, the likes of Dead Money was enthralling while offering a level of tension you just didn't get from the other parts of the game. A very easy title to return too, especially once you get into modding things.
Kingdom Come was a breath of fresh air in many respects, a beautiful game with plenty of humour and things to do, but a little more grounded in reality compared to most games of its type. The combat was punishing if you tried to rush in like a super hero, taking on a camp of bandits requires strategy, although one should never underestimate a mounted archer! Playing an RPG that tried to be more true to life was definitely an experience I'd like to see repeated, as much simulation as RPG and it gave a very interesting dynamic. Another game that felt more like a natural, living and breathing world.