Hard for me to pick a single game, but I think can narrow it down to a top 3 and justify each. So, without further ado...
-Fallout 3/NV
In the case of FO3, it was my first Fallout and from the very get-go, the story, gameplay and "next-gen" graphics had me hooked - as did the epic quest to try and find some of that awesome power armour from the BoS member on the cover art! Hundreds of ours, scenarios, roleplays and builds played-out and not a single one regretted. I even cosplayed as a female merc from it. (Kind of lame, I know, but I loved the outfit.) As for FONV, the story was a little bit of a step back, as was the lack of combat by comparison, but I vastly enjoyed the gameplay -- higher damage values, rarer and more expensive loot, etc. Again, hundreds of hours to the same effect. ...This was before I modded either game as well. Modded FONV remains one of my favourite experiences with videogames.
-Grand Theft Auto Online/GTA V
While I was not a huge fan of the single-player aspect of things, this remains my most-played game of all-time, by a loud, brash (and almost pitiful) mile. I absolutely adored GTAO from the first day I stepped into it after finishing story mode. Granted, it has gone to shit now, but I still enjoy the core game itsself, if you can look past the cash-grabs and greed.
-NieR: Automata
This is my most recent of the bunch, but truely one of the most unique, gripping and gratifying games I have ever played. Simply put, it is stunning; in graphics, variety, value for money, story and style. Admittedly, the game brought me to tears thanks to it's gripping story tugging on the heart strings and I am not fully done with it yet, but it has been such an enjoyable experience that it bumped up to my best game of 2017. I am not usually one for Japanese games even, but it is astounding.
Synopsis
So, those are my top three, for varying reasons, but all of which hit pretty close to home and my great videogame experiences. I hope it was an enjoyable read. I could throw some shout-outs to the original Deus Ex, the Motorstorm series, Full Auto 2, Unreal Tournament 99, the entire Half Life series, as well as Doom 2016. (Seriously, Doom 2016 was fucking awesome and my top game of 2016.)