Definately the bumblebee catfish. And they don't grow, either. Little 2-inch, invisible stinkers. We have two, and we see them about once every 2 months when we rearrange the tank.
My shyest fish would deffinately be my clown loach. He was so shy that he hid in a ceramic ornament and wouldn't come out ever, so i broke the thing open, got him out and dumped him back in...lol now he still hides, but he comes out every once and a while...he doesnt wanna have to expierence the hammer again lol.