Hard to say, but I love james Horner's score even if it uses alot of Star Trek II in it, what do you exspect for the amount of time he had to write the thing.
Jerry Goldsmith is the next best, it sucks that they havent released a delux version for his Alien Score.
Elliot Goldenthal very dark soundrack, and it fits the mood of the movie.
Brian Tyler, score was great. For me it is what held the movie together, the main theme is awsome.
Harlold Kloser, Had great themes as well, Alien fight, and down the tunnel being the best.
John Frizzel, Good score as well nice main theme.