Doom, because it was the definitive FPS, and almost 20 years on there's still nothing in the genre to rival its gameplay.
Fear Effect and Fear Effect 2. In the PS1 era when ingame facial animation was basically unheard of it used cel shaded characters (and brilliant voice actors) to create genuinely expressive characters using the in-game engine rather than CGI.
Minecraft, because it's hugely innovative and the most incredible exploration game ever made. Despite the blocky graphics no other game comes so close to convincing you that you're in an entire other world.
Fear Effect and Fear Effect 2. In the PS1 era when ingame facial animation was basically unheard of it used cel shaded characters (and brilliant voice actors) to create genuinely expressive characters using the in-game engine rather than CGI.
Minecraft, because it's hugely innovative and the most incredible exploration game ever made. Despite the blocky graphics no other game comes so close to convincing you that you're in an entire other world.