Vortigaunts definitely take the #1 spot. They're interesting, helpful, and they shoot LIGHTNING from their fingers. Also, they saved Alyx's life twice, which is more than a good thing. Special mention also has to go to the Irkens, who are so completely ridiculous and incompetent that they come full circle and become awesome again. They're also helped by the fact that their highest-profile member, Zim, was pretty much the most gloriously hammy character in all of fiction until Cave Johnson came along to steal that title.