My vote is for the Alien, partly because that's what it is - alien. It's got no eyes, for one thing, which means that its entire perception of reality is likely to be very different from our own. The acid blood suggests a very interesting biochemistry, the inner jaw mechanism is downright weird, and of course there's the life cycle.
There are also thematic aspects of the Alien that I appreciate. On an individual basis, the life cycle has an element of body horror, and the species represents infection and pestilence both on an individual level and on the level of an entire infestation. And of course, eusocial species and hive-building species are always interesting on their own.
