Aliens, because it's Cameron's only truly great screenplay. The screenplays for T1 and T2, The Abyss, Titanic, etc., are fine enough, but the dialogue in Aliens is 100% pure brilliance (compared to the dialogue in, say, Titanic, which is crap).
The genius thing about the Titanic screenplay (and why I think it's underrated, even in spite of the crap dialogue) is how good it is at teaching you all the stuff you need to know about the ship in order to understand what happened to it after it struck the iceberg. It's easier to watch if you realize that Cameron really doesn't give a shit about the romantic subplot: that's just the narrative hook to use as an excuse to give the viewers a tour of the ship, tell us there aren't any lifeboats, that people of different social classes are housed on different decks, that the water will induce hypothermia, etc. He has to communicate all that stuff in the first 100 minutes or so, and he's expert at that (though at the expense of having any decent characters).
The genius thing about the Titanic screenplay (and why I think it's underrated, even in spite of the crap dialogue) is how good it is at teaching you all the stuff you need to know about the ship in order to understand what happened to it after it struck the iceberg. It's easier to watch if you realize that Cameron really doesn't give a shit about the romantic subplot: that's just the narrative hook to use as an excuse to give the viewers a tour of the ship, tell us there aren't any lifeboats, that people of different social classes are housed on different decks, that the water will induce hypothermia, etc. He has to communicate all that stuff in the first 100 minutes or so, and he's expert at that (though at the expense of having any decent characters).