Half Life has been my favorite game since I was a child and got my first PC, a Pentium 4! The game was amazing, and the story really good, I enjoyed every second playing Half Life and the rest of the franchise.
Quake II - first FPS I played that was easy to get friends together to do LAN parties. Before that we spent more time faffing around with BNC cables, terminators, network cards, drivers, IRQs etc only to find out that someone didn't have the game installed or enough disk space/memory or a good enough monitor.
My all time favorite game is mass effect andromeda, its an pen world and you can travel different parts of world with different kinds of environment and enemies. I hope they would make another part of the game soon.
I gotta say it's Fallout New Vegas. It's one of those, if not one of the only, games where you can be ANYONE armed with ANYTHING, doing EVERYTHING at your disposal with or against EVERYONE. The power that your decisions carry affects everything in a manner so powerful, even the Expansion zones react to what you did on the Mojave Wasteland. The roleplaying, even if the game is now so outdated, its more complete than in new iterations of the series, or completely different series.
Such an excellent game, even if it was rushed and on a crude engine. Not even those flaws can destroy the core of this masterpiece.
If you can, but it 10 times on each sale, or better yet, when it isnt on sale. It truly is a must for everyone
My all time favorite would be the original X-COM: Ufo Defense. No game can replicate the thrill and tension of the alien encounters, not even the new reboots can compare to the original. And this game actually introduced me to PC gaming which I still enjoy up to now.
Escape from tarkov... It's very new, officially its unreleased but I've been playing 1.5 years, and it fulfills everything I want in a game. Challenging, rewarding, detailed, expansive, and continuously growing.