Pierson's Puppeteers, from Larry Niven's Known Space booksRead about the other entries on the list.
These aliens get points for originality and not simply being humanoid. As Tim Morris explains, "Its biology is singularly bizarre, it has 2 heads (each with one eye, one mouth, and three lip-fingers), neither of which has a skull or brain. The brain and skull are actually on top of the rib-cage, at the base of the 2 "necks", this essentially makes the "heads" simply glorified elephant trunks or eye-stalks. Also, they have three legs, 2 front, and one back." But a number of people singled out these creators of insane high technology as not really holding up. Says Sean Toleson, "They don't have hands to make things, and are consummate cowards."
Also see: Alice-Azania Jarvis's reading list on aliens and John Mullan's list of ten of the best aliens in science fiction.
--Marshal Zeringue