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
the best Pokemon to pick at the start is PIPLUP or CHIMCHAR
There are all pretty good Pokemon but my opinion is Chimchar. He evolves into fire/fighting type.
You cannot find a Charmander on Pokemon diamond. You have to get it from Pokemon Fire Red or Pokemon Leaf Green. Charmander is a starter Pokemon, so you don't have to catch it.
As Totodile is a Generation 3 starter Pokemon, it cannot be obtained directly in Pokemon Diamond. Instead, players have to transfer it via trades from previous generations of the Pokemon games.
Chimchar is a starter pokemon so you can choose him with Barry at the beginning.
Chimchar
the best Pokemon to pick at the start is PIPLUP or CHIMCHAR
piplup is the best starter pokermon piplup is the best starter pokermon --------------------------- I chosed Turtwig grass type
The starter pokemon for Diamond and Pearl are Turtwig, Piplup, and Chimchar.
in order from best starter to worse Piplup Turtwig Chimchar
Choose it as your Starter Pokemon.
You have to choose it as a starter
piplup
you can get it as your starter Pokemon
I'm pretty sure that the only way to get another starter Pokemon in Pokemon diamond is to go to the Global Trade Station (GTS) and trade with someone.
A level 60 or up starter Pokemon dialgua or palkia and any Pokemon over 60 just not graveler
Turtwig in Pokemon Pearl and Diamond.