Pokémon with perfect IVs (Individual Values) in every stat. Considered the holy grail of breeders and RNG abusers, the potential such Pokémon have are ultimately limited by what species they are and movepools they have. Very few Pokémon have the base stats that can take advantage of every stat (Mew and Arceus, which have even stats of 100 and 120 respectively) and even they are limited because a Pokémon can only know four moves at a time. The amount of Effort Values a Pokémon can have caps at 510, and you can only max out 2 stats to the fullest. It's pointless to have a 31 (the highest IV) in Attack when your 'mon is a special attacker (and vice versa), and having a 31 in Speed is detrimental to Pokémon that need to move slower than the opponent (To take advantage of Trick Room, make use of Gyro Ball, or successfully set up their weather against an opposing weather setter). Even having 31 IVs in Defense and Special Defense doesn't work as well as it should if said Pokémon has subpar defensive stats, bad HP, or a bad defensive type in general that gives it a ton of weaknesses. Limited Effort Values once more add to the issue that this may potentially cost the Pokémon more attack power or speed if they are used on its defenses. It's pointless to use defensive EVs on a sweeper or wallbreaking Pokémon, the former who will need all the speed it can get. No single Pokémon can really do it all.