Which ones do you want? What kind of alien encounter are we talking about here? The Qin are slugs in advanced bio-tech that are kinda friendly towards humans, the Chromatics are (right now) in a tribal society, the Doyen, cowardly blue ectoplasmic snotballs that they are, can't go anywhere near Earth without exploding, and the Coalition wouldn't make bad enemies assuming you don't mind introducing an enemy that can wipe out planets. Of course, these are just the canon races; you might want to consider making up some of your own, and you should be liberal in theft. Try making the Mi-Go from Call of Cthulhu, or high-tech versions of the classic D&D Illithids.