As far as games go that I played the most:
DeusEx (2000) - This game should be obligatory to know for every game developer, it lays down the rules of game design 101 that every game should adhere to (sadly most does not, or only to a few of them) despite finishing it dozens of times, it still has secrets I'm yet to uncover.
Mass Effect 2 (2010) - It is the most well rounded game I played, storytelling, gameplay, characters, it all comes together.
XCOM2 (2016) - it's addictive, I've played it over 1100 hours and counting. With mods I can go on forever probably. Since most encounters are procedurally generated it never gets repetitive. Despite the fact that I've probably restarted the campaign over 100 times.
Mafia: City of Lost Heaven (2002) - The game that brought together two of my favorite game genres, driving simulations and story driven action games.
BeamNG Drive (? - Still Early Access) - Sure it's hard to call this a game yet, but for me as far as I can remember crashing cars in spectacular ways was always fun in of itself, and currently this has the best and most realistic damage model of all available driving games.
Gran Turismo 4 (2004) - The best the series has ever been, sadly it's been all downhill since then, with sports being rock bottom
But there is another category of favorites, that I might only have played once, but that 10-20 hours was such a ride that I'd not have wanted to miss them at any cost.
The Last of Us Part II (2020) this is a very recent title but it quickly found its way to the top of my favorites.
Beyond: Two Souls (2013) The game itself might have been dreadful, but the story and the sense of control you had over the fates of characters made this a thrill, I actually feared for the well being of fictional characters in a game.
Alien Isolation (2014) - Fear itself, when it's not super scary, it's creepy, but danger always lurks around every corner, the atmosphere of this game is so ominous, that I think it beats the actual movie it is based on.
Call of Duty Infinite Warfare (2016) - It's only fitting that the most universally hated instalment of the series would be my favorite. The contrarian in me rejoices. Peace to the fallen - the only time an end credits made me cry.
Alpha Protocol (2010) - This is an objectively bad game, with serious flaws, but it gets some things right, that no other game did, it must be experienced at least once for this
Splinter Cell: Chaos Theory (2005) - This is more of an honourable mention, because this is to appreciate the whole Splinter Cell Franchise with all its lows and ups, but this is the best part of the series so it gets on the list.