it has been formally suggest before that the an appropriate message signifying intelligent origin is an equilateral triangle. something about the unity of mathematical beauty, and the fact that it does not occur in any precise form in nature. i'm not sure that latter bit is true, but the argument wasn't made by me.
probably not coincidentally, one of the original planned shapes for the iconic monoliths of 2001: a space odyssey was a tetrahedron (a perfect equilateral triangular solid). in the end, they went with a 1:4:9 rectangular sold -- the proportions representing the first three perfect squares.
i'm not sure either is definitive proof of intelligent origin, but it's certainly better than anything id has to offer.