The Top 7... alien invasions

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.