Which Pokémon would be the most useful IRL?

Sure, these three would be useful, but I think the consequences should be considered. The more money is in circulation, the lower the money's value. If everybody had a Meowth, for example, money would pretty much be worthless. Arceus has all the power in the world to do whatever it wants. If you get on its bad side, it might very well be the end for you. Eternal life seems like it wouldn't have a downside, but you'll eventually run out of things to do, witness the death of all your friends and family if they're not affected, you'll live through the destruction of Earth, stuff like that.

You get a Pikachu to ghostwrite for you. Of course, the book may end up consisting of only the word "Pikachu".

miraidon because it would help Sisyphus roll a bigger boulder

I love Dragonite! It's loyal, strong, fast, big, huggable, friendly, it's a free ride, it can fly, it's a literal dragon, and you're telling me that if I don't make it angry, it's not even going to have any consequenses? I really need one of these.

rockruff after it evolves