1. Half-Life (1998, PC)
It wasn't until the late '90s that the pure physical terror of an alien invasion could be communicated in a game. Half-Life captures the utter chaos of a world torn to pieces by aliens pouring into it - and destroying everything in their path. The detailed creatures and their advanced behavior was entirely fresh, and still stands as a high watermark. The epic battles against the invasion forces from the world of Xen are unsettling and extreme.
And, eventually, you become the invader - taking the fight to the alien world. It's a game that managed, through the application of quality writing as well as convincing graphics and gameplay, to really communicate the truly disquieting possibilities that lie at the core of contact with a species that has nothing in common with humanity.