I echo Mirror's edge - loved that game.

Also, the original Dungeon Siege: Legends of Aranna was a solid entry for its time and I hold a lot of nostalgia for that game.

For me, though, my childhood was the Sierra Impressions city-building games: Pharaoh and Cleopatra, Caesar III, Zeus and Poseidon... I spent so many hours in my teens building cities and exploiting my workers. Caesar III still stands out as a masterpiece of the genre.