There's no way to decide the smartest Pokemon since they're fictional creatures, not real. However, we can decide what Pokemon are really smart, from their Pokedex entries and their behavior in the anime.
It has an incredibly high level of intelligence. Some say that Alakazam remembers everything that ever happens to it, from birth till death.
—Pokemon Sword's Pokedex entry for Alakazam
Known as "The Being of Knowledge." It is said that it can wipe out the memory of those who see its eyes.
—Pokemon Diamond's Pokedex entry for Uxie
Metagross has four brains that are joined by a complex neural network. As a result of integration, this Pokémon is smarter than a supercomputer.
—Pokemon Emerald's Pokedex entry for Metagross
A poisonous bite reacted with its system, blessing it with the enhanced intellect of a genius. It has full control of its psychic powers.
—Pokemon Sun's Pokedex entry for Slowking
With waves of its fan—made from leaves and its own fur—Oranguru skillfully gives instructions to other Pokémon.
—Pokemon Sword's Pokedex entry for Oranguru
Latias is highly intelligent and capable of understanding human speech. It is covered with a glass-like down. The Pokémon enfolds its body with its down and refracts light to alter its appearance.
—Pokemon Sapphire's Pokedex entry for Latias
Latios has the ability to make its foe see an image of what it has seen or imagines in its head. This Pokémon is intelligent and understands human speech.
—Pokemon Ruby's Pokedex entry for Latios
It has strong psychic powers. Using its fingers that flash three different colors, it controls its opponents and rewrites their memories.
—Pokemon Ultra Moon's Pokedex entry for Beheeyem
Calyrex is a merciful Pokémon, capable of providing healing and blessings. It reigned over the Galar region in times of yore.
—Pokemon Sword's Pokedex entry for Calyrex
These remarkably intelligent Pokémon fight by controlling arms that can grip with rock-crushing power.
—Pokemon White's Pokedex entry for Reuniclus
A constant collector of information, this Pokémon is very smart. Very strong is what it isn’t.
It’s famous for its high level of intelligence, and the large size of its brain is proof that it also possesses immense psychic power.
These Pokemon are probably the most smartest ones. Comment if anything is missing.
Hope this helps!