the mythical Pokémon Arceus (yes, it is mythical, not legendary, because it was obtained from an event) is the strongest base stat Pokémon. While many other Pokémon are almost tied for the top spot, for some reason, I always think Charizard is the strongest, even if that’s far from...
Gyarados itself is incredibly strong with Strength after gym 3, but does kind of fall off lategame due to its lack of physical STAB moves. Earthquake can help keep it somewhat more relevant than RBY attacker nido, but obviously Horn Drill lets Nidoking participate lategame too. Torkoal ...