This one's gonna be a doozy.
Galactic Civilizations 2 and 3 - It's like Stellaris but simpler, has a more flexible ship customization system, and is filled to the brim with jokes/parodies of classic and modern sci-fi while still taking its story seriously.
Star Wars: Empire at War - First RTS I've ever played. It's simple, but fun, and mods/continued bug fix updates keep it alive to this day.
Star Wars: Jedi Outcast - One of the few star wars games out there that really hits my inner nerd and makes me feel like I'm on a classic star wars adventure. Developed on the ID Tech 3 engine, its shooter and movement gameplay feels right at home with 'boomer shooters' such as doom, quake, and half-life, but where the game really shines is its force powers and lightsaber combat. Fallen Order/Survivor don't hold a candle to this game in my eyes.
The Half-Life/Portal Games - Source Engine games were my DOOM, pretty much. No matter how old these games get I always find myself coming back to them to at least mess around with the numerous cheat codes (There's so many console commands for source engine they needed a whole wiki for them.) or try out a new sourcemod.
The Zeno Clash series - One of the most unique and surreal fighting games I've ever played, with a surprisingly deep story, it's always fun to come back to.
Sacrifice - An RTS I love for the same reason I love Zeno Clash: A unique surreal artstyle, which takes the classic DnD/Tolkien fantasy setting and warps it into a dark, twisted land filled with abominable creatures and the ambitious wizards and spiteful gods who rule over them. The characters are all charming and fun, and there's tons of different story paths.
The Elder Scrolls games - Goes without saying.
The Halo Trilogy + Reach - Also goes without saying.
Mass Effect - Also goes without saying.
Scrapland - An obscure open-world thirdperson shooter, it's about a post-apocalyptic Earth that's been abandoned by humans, and resettled by the countless robots they left behind. It's basically Grand Theft Auto but with robots and fighter ships, and has great art direction. Made by a small spanish studio, and amazingly its english localization is pretty good despite their localization 'team' consisting of just one guy.
Grand Theft Auto 4 - It might not be as feature heavy as the other GTA games, but this one's story, music and atmosphere hits just right imo.
Bully - I come back to this game a lot because it's just so damn fun.
Darkstar One -An obscure space combat game, and one of its developer's last games before they went bankrupt. Its controls are simple and there's lots of upgrade options for the player ship, so I often come back to it when I feel like trying a new build.
Deus Ex - This game was hard for me to get the hang of at first, but my god did it grow on me fast. Every time I play it, my mind is still blown at just how detailed and open each map is, especially for the time period it was released.
SiN - A cyberpunk FPS from 1998. I still occasionally come back to it for its fun gameplay and badass rock music.
Postal 2 - What's there not to love about drowning your next-door neighbor in piss and napalm?
Titanfall 2 - One of the last good shooters of the 2010s, came out right before the gaming industry's decline went into full swing.
