The cheap answer would be Palkia. Then we could just transport ourselves back to the Pokemon world. Why go through all the complicated mess of trying to recreate this world with Arceus when going to their world would be so much simpler?
If those shenanigans are impossible, then Ninetales. Besides being my favorite, it can breath fire, put people to sleep, stop rainy weather (great during severe storms), place curses on people, and so much more. Oh, and it's fluffy.
People already said it: Arceus. Then I'd show him the games, the entries, and talk about what is good and what's not and try to make a world of pokemon that doesn't have some of the more life-threatening or tragic entries, or even the entries that simply don't make sense, etc. And maybe once everything's over, nerf legendaries so the world isn't in peril every other week.
As for an answer that isn't a cop-out, Salamence w/ a Jolly nature because they'd make for a great companion.
Other than the obvious "Arceus, since he can then create the Pokémon world" answer, I'd have to say Eevee, if we're counting evolutions. If we're only counting one stage, then I guess maybe Miltank? I think they're cute.