This is across the entire series.
I haven't played any of the DLCs, but Hard Rain is easily one of my favorites. Just, the unique stuff man. Having to plan your trip back, dealing with the rain, THE SUGAR MILL.
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L4D1: Death Toll and Blood Harvest L4D2: The Passing, Dark Carnival, and Hard Rain. Though I like all the campaigns of both games.
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Hard rain was awesome
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Left 4 Dead 2: Case Zero Why? Because its -blam!-ing short and sweet. The length of other Left 4 Dead games pisses me off.
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Hard rain. Love the challenge.
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Parish.