Is an IQ of 5000 even possible? Has the IQ system stated a limit where you can go?
There isn't a set limit, but so far, the highest to have been thought to exist was this American guy who was believed to have an IQ of bout 291 or so...
While 5000 sounds extremely unrealistic, no one could say if it was possible... IQ is basically just how fast the brain can work and process information... This would mean, however, that the artificial Pokémon, whose brain is basically a supercomputer, would probably have an IQ that heavily exceeds humans' and most, if not all, Pokémon...
However, I agree that the naturally smartest Pokémon would be Slowking(it's the only Pokémon capable of talking the human language), Alakazam(due to its unrealistically high IQ), or Mewtwo... I've noticed many people say Mewtwo isn't a "real" Pokémon, but if you follow the games, Mewtwo was birthed by Mew, as previously mentioned, and is actually the ONLY Pokémon born and not hatched from an egg.
And well, if we imagine that IQ increases Psycic power, Mewtwo's would probably far exceed Alakazams... For example, if we go by the Anime, he was able to use his Psycic powers to create storms thought to be "powerful enough to destroy the planet"