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.