Before gen 6, only Arceus was the one uber pokemon who could've been deemed truly "broken",and even then it's not like he fucked up the metagame.2 gens later we got shit like geomancy Xerneas, primal Groudon and Kyogre, and a lot ot megas for pokemon who didn't need any sort of powerup.
Imho gen 6 is when they broke the camel's back in terms of competitive play, and stuff like z moves and gigantamax forms got nothing on anything from gen 6.