I have heard Libras have the best smiles (and, yes, I read that in an astro profile). I would think that Taurus would have good smiles, too (also being ruled by Venus).
i'd say scorpios have the best smile because they rarely ever smile, at least the ones i run into. and when they do smile, its golden, and very charming. its a shame they don't smile more often, but then again, i guess it wouldn't be as precious if they handed them out frequently, huh?
cancers have nice smiles too, but nothing out of the ordinary for me. sags and aquas have very contagious smiles, they make me want to smile with them even if i'm in a pissy mood.
I here cancerians have the best smiles.