Wait a second, I'm forgetting Ultima IV.
Shaped my current thinking on video games as well some things not related to games at all. During the course of the game, the player had to adhere to strict virtues - had to be kind, honest, caring, selfless, etc - to succeed. Basically invented the whole "Good/Bad" system, punishing the player for being bad by not allowing them to proceed.

Finally figured out what I like about Metroid: the storyline was very open-ended, and as opposed to just finding missions you had to explore this entire world, revisiting places you had been to and uncovering new items each time around. Never seen a "flow" quite like it. Closest thing was Zelda, but even then there were strictly defined "dungeons" that became relatively moot after you played through them once. Also, the environments were REALLY cool. I could spend hours just wandering around the Sanctuary Fortress in Prime 2. In fact, my only real complaint about Prime 2 was that the weather before you messed with the energy controllers always looked better than the wather afterward.

Lucasarts: X-Wing and then Tie Fighter. Spent endless hours on those games. I wish they made more games like those.