Hmm...out of the Alien franchise, I definitely have to go with Aliens (Alien 2). It introduced the entire concept of the xenomorph hive, which in my opinion was absoultely stunning, and the plot was much more well-developed than any other film in the series. The acting was superb, and what was done in that film far exceeded other movies of its era.
As for Predator, I would have to go with Predator 1, simply because it is the original, and because the entire concept of the movie was so simple and yet so enthralling.
Between the two, I must side with Aliens ^_^
And thus I clothe my naked villainy
With old odd ends stolen forth from holy writ
And seem a saint when most I play the devil.