To narrow the scope down a little let's exclude Chewbacca, Yoda, Jabba, Maul and Grievous (though the former is more droid than alien anyway) just to give some other characters some more limelight. Now I know that when we watch the movies very few alien characters are explored in depth, some are more prominent than others (Dexster Jettster, Greedo, Bib Fortuan, Jawa's etc) whilst others are seen in only passing glances (insert any alien extra) Some have EU backgrounds but we're not concerned with them, only their movie appearance. So my favourite alien character would have to be Admiral Ackbar, no other has ingrained such a distinguishable persona in my mind, with his goggly like eyes, fishy voice and one permanent face expression (surprise) its a real wonder how anyone took this fish face seriously. As if his own starship and fleet weren't enough, his own floaty chair (just like Yoda) makes this guy even more cool. But of course (just as you expected) Admiral Ackbar would be lost in obscurity amongst the many denizens of the Star Wars universe if he had not uttered those 3 golden words, not only stating the stark naked obvious but spelling out the dire situation for all the Rebel alliance and unintentionally prompting a chorus of "It's a trap!" followed by rapturous laughter and several rewinds just to watch it again and again. Oh and he's got his own breakfast cereal too, not bad for the old fish face eh.