What Pokemon is the best Starter for Diamond?

Well they're all great

In my opinion its Piplup, because the first Gym is EASY PEASY if its level 15 or evolved.

But if you started with Piplup pick up a Starly and train it up into a Staraptor because Staraptor is AMAZING it learned Close Combat when it first evolves but anyways.

Piplup is slow, so get the Quick Claw from the lady with the Pachirisu and Pikachu in the Pokemon Condiminiums in Jubilife.

Plus is you started with Turtwig too, The Quick Claw is nice for it too because Grotle is pretty slow too.

Chimchar is Ok, but its not good for the Elite four because i think its weak to Bug, Ground, fire, and Psychic so its not a good choice but if you like fire types i guess its fine.

Turtwigs final evolution it FABULOUS for anything it learns Earthquake right off the bat after it evolves into Torterra. So either Turtwig or Piplup is Perfect.

Plus Empoleon (piplups final evolution) is my favorite Pokemon.

Hope it all helps