The egg, of course. Because preceding the first chicken must necessarily have been a pre-chicken chickenlike bird, the immediate ancestor of the chicken, which is not, itself, a chicken, but is almost exactly like a chicken. But as a matter of physiological fact, the egg always forms before the sperm arrives to impregnate it. So there is no chicken until the unchicken mother gets impregnated by the unchicken father and their sperm and egg combine and mutate, producing a chicken baby, which then grows up to mate with other unchickens, randomly producing chickens and unchickens, and so on. If the chicken mutation is advantageous, over time, the unchickens dwindle in number and eventually go extinct, leaving only chickens. The actual mutation starting all this may have begun in the sperm of the unrooster or the egg of the unhen, but the effect of the mutation (a chicken) would not occur until both were combined and actuated, generating a fetus.
At least, that's the boring way to put it.