"I believe that it would be any Olympian god. Remember, Zeus was just a surviving child of his father, after his father devoured all of his children, in fear that they would do exactly the same to him as he did to the creator god, however he freed them and they became the other major gods. I assume, that if all of the gods disliked Zeus, they would rebel and throw him into tartarus, thus crowning a new king of the gods. Gaia would aid a would be king of the gods too, for she has already tried to banish Zeus, when she let the titans invade the heavens, even though they failed, perhaps something else could happen which is similar. All gods seem to have the blood of Titans in them anyway, which makes it possible to fight them, if one should have enough power. Zeus was born of Cronos and Rhea, a god and a titan, Who was also his sister, (A lot of inbreeding with the gods...). The gods seem vulnerable to weapons made of flint or adamantine (I think that is what it is called), this is what the sickle was made of, when Zeus attacked his own father. Give your new god such a weapon and he could harm Zeus, though unless he/she either attacks in secret or has the support of the other gods, they will not get very far."