My vote definitely goes to the predator. To me the aliens serve as a good monster, but they're just slavering beasts - the predators have personalities, a culture and homeworld, choices to make, a code of honor. Not to mention I like getting to see different designs, both physically and in regards to their armor, equipment, etc. We get enough information about them to make them compelling while also being free to fill in the blanks with what we'd find interesting.