Logically, and assuming the Darwinian theory of evolution, it must have been the egg. An bird that was similar to, but not quite a chicken, would have lain an egg, which would then mutated, according to genetic variation, and became a chicken. Thus the egg came before the chicken.
Also, if one thinks very abstractly, other birds existed before chickens, and those birds would have lain eggs long before chickens.
Either way, the argument strongly leans towards egget. ☺