Half-Life. Other people already explained why.
Thief. First First-Person-Sneaker.
Morrowind. First RPG I played.
Far Cry. Great mix of stealth, freedom, great AI and the physics (by the way, six months before HL2) and the graphics came as a great bonus (yay for polybump). Also, I liked the Trigens. Though the story wasn't that good. Oh well, can't have everything.
Portal. Other people already explained why.
Saevus said:
How is Morrowind breaking the mold? Look up Daggerfall. Point out some games that actually did something really new and exciting and (in general)failed utterly because they didn't suit the zeitgeist.
"I'm interested to find out what games struck
you through your gaming years as different and unique in terms of art style, gameplay mechanics or simply because they were different from the rest."