8. The Flood – Halo: Combat Evolved
Despite being essentially a bunch of plants, the Flood managed to steal the show in Halo: Combat Evolved. We're thinking specifically of the infamous Library stage, where the virulent parasite caused us to mess our pants and scramble for the nearest exit.
The Flood made an indelible impression on gamers; mainly because of how out-of-the-blue they were. One moment you're battling alien soldiers in a typical warzone setting -- the next, you’re locked in a room being pursued by jellyfish-like critters and zombies made from bits of your former allies. It was completely unexpected, and subsequently threw the plot into a tailspin.
Those that lived to tell the tale can attest to the level's difficulty (as can the numerous tutorial videos on YouTube…)
As players that, by sheer luck or superhuman skills, have passed the Library can tell you, The Flood is one mean mother.