Marine, by far. It was the most tense, action-packed and scary of the three by a long, long way. It was also the longest - I hated how short the campaigns were in this game.
I can't believe people are actually saying the Alien campaign was the best. It was terrible, by far the worst of the three. It had no plot whatsoever, it was just 'go here, kill these people, go here, kill some more people'. There was no substance to it at all, it felt like a tacked-on afterthought.