some people have said that turtwig is because it evolves into torterra and it is a Grass/Ground type, but you have to take into considerasion that Chimchar evolves into Infernape which is a Fire/Fighting type and piplup that evolves into empoleon which is a Water/Steel type.
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.