Alien species diversity is one of my favorite things about science fiction.
Let's face it, otherworldly beings, aliens, extra terrestrials, xenoexotic lifeforms is the one unviersal difference between a drama or action or comedy story set on Earth in the "real world" and the same exact story set elsewhere off Earth. In MY opinion, the MORE diverse a location is with the most variety possible, the better.
So what are your favorite species of aliens? Sure, I'm especially asking about Star Wars but all other sci-fi properties certainly apply too: Star Trek, Dr. Who, Stargate, DC comics, Marvel comics, X-Files, Ghosts of Mars, EVRYTHING ELSE?