Metroid Prime 1 for me. I just love the story of Metroid as a whole and I bought a gamecube just because of it. The graphics were ahead of the time and the solidarity of the enemies and the creepiness of the setting really accent the feeling of being all alone on a planet against basically everyone. Beaten it once and never played it since to keep the memory of it intact so I won't judge it to today's FPS games. Great bosses, crisp settings, and sweet weapons. Honestly nothing like it.