What are they, and why? Don't lie to me - you know that there are games, quite a few from your teenage years, that you'll never forget, and always cherish. Mine:
1) Half Life (and Counterstrike) - fused action and adventure in an FPS in such an awesome way. Had seen nothing like it before this!
2) Grim Fandango - not sure how many of you touched this, but it was like a top class film. Incredible atmosphere, characters and writing. Unrivalled in its hilarity.
3) Unreal - the atmosphere, the atmosphere!
4) Dizzy (Commodore 64) - you played a fucking puzzle-solving egg!!!! Hidden coins in magic clouds. What more can you want?
5) Age of Empires 1/2 - nothing better than setting up your english longbowmen well outside the range of the enemy's shitty seige weaponry. Also, using your priests to convert men from a distance! Holy shit, literally!
EDIT: How the fuck did I forget about Morrowind!? Talk about atmosphere in a game! I actually wanted to be transported there!