Oh gosh, that's a good one.
Half-Life, because the cinematic storytelling experience blew me away. I was late to the 3D video game thing & this is essentially what got me into computers...I needed a video card to play it & had no idea what that even was, down the rabbit hole I went!
Portal, because it was just so much fun.
Mario, and if I had to pick, probably Super Mario Brothers 3. I still play them all the time with my family, never gets old!
Counter-Strike, that was my first intro to LAN gaming. A lot of fun online, even more fun at a LAN party when you can yell at each other in person 😀
Castlevania series, love 'em, especially the old console versions, no matter how impossible some of them were. And of course SOTN because wow, the graphics on a platformer at the time.
Serious Sam, because I love FPS games & it's just so silly.
Twisted Metal 2, many good times spent with siblings & friends growing up blowing the crap out of each other.