Rocket Flood

I personally think 343 Guilty Spark is more of a horror level. Its shrouded in mystery and that cutscene with Jenkins (80% sure that was the marine's name) really set the tone. When first playing through it you really didn't know what was coming and you had yet to be introduced to the flood. And just things like blood down the elevator shaft and seeing dead bodies heading in, but the bodies are missing heading out make it a masterpiece of a level from a storytelling perspective.