Porygon Z

While it's true that after Z-Conversion Porygon-Z becomes a threatening sweeper, its overreliance on stat boosts from the aforementioned move, inability to get a second chance at sweeping, and reliance on a lot of support hold it back from being viable in the metagame. Furthermore, Porygon-Z's low stats, except Special Attack, mean that it's hard to find setup opportunities due to low defenses and lack of resistances, so Porygon-Z is outclassed by better setup sweepers like Magearna. Even after setup, Porygon-Z is still relatively easy to revenge kill with common forces like Choice Scarf Landorus-T and Ash-Greninja because Porygon-Z doesn't have priority to hit these revenge killers.