One opinion is to go with Turtwig since it can learn Earthquake and Absorb however another option is to pick Chimchar since its Fire-type attacks are super effective against the Grass, Steel and Ice-type Gym Leaders and Chimchar can also learn Fighting moves which will help it to beat the Rock-type Gym Leader.
It depends on your preference.
infernape is good against the first 2 gyms, while the other two arent, which is good cuz there arent many good Pokemon right off the bat. My favourite is Torterra, then Empoleon, then Infernape. Torterra is my faveourite because its has good defence although its slow. Also when it learns earthquake it will be against fire. XD
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
piplup
you can get it as your starter Pokemon
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
they are chimchar, piplup and turtwig