I'm going to go with one that was only mentioned once but not really talked about: When the Flood shows up in the original Halo.
I'm sorry, but when I first started having those little crawly, thingies coming at me and then the marines...Jesus. I swear, I just ran like hell through the hole thing, going OH SHIT, whenever I saw another jump FIFTY FUCKING FEET to land RIGHT IN MY GODDAMN FACE>
I literally had to pause and regroup myself about four times during that mission.