Gaila The Orion Slave Girls of the original Star Trek series are the prototypical hot alien girls.
No list of alien girls would be complete without this grey skinned, white haired thief played by Gigi Edgley.
Here's Fisher on the set of Return of the Jedi catching some rays is her bathing suit.
Where does she find such products on the sandy planet of Tatooinne? The characters come from movies, video games, comics, and more, and the list is limited to female or female-analogue biological entities from a different world than the main protagonist.
Aeryn Sun from Farscape also deserves a mention.
Hot alien girls have been a staple of science fiction for nearly as long as the genre has existed.