What's the most believable alien in science fiction?

We've all seen tons of aliens in movies, television, comics and of course novels. But some of them seem more way more plausible than others. Both physically, in the sense of having evolved on another planet, and in terms of having a culture and society that seem fully realized. What's the most believable alien in science fiction?

Please include a picture of this super-believable alien, so we can all see for ourselves.