Piplup.
Piplup is the best starter. Its final evolution form - Empoleon is strong. Its extra steel type decrease the super effective damage from lightning type to water type. And it can learn some good moves too. Like Drill Peck, Hydro Pump, Avalanche, Hyper Beam, Giga Impact and many more. What's more if you want to teach him HM moves, well he can learn all of them except HM02 Fly.
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
you can get it as your starter Pokemon
piplup
Choose it as your Starter Pokemon.
You have to choose it as a starter
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.