What should you choose as a starter in Pokemon platinum?

It's what ever you prefer mate. Fire, Water or Grass. I can put all the starter Pokemon in the WiFi Grand Trade System if you want, so you can get them all including the other starter Pokemon from all the other versions. Like Charmander Bulbasaur and Squirtle.

When I get a Ditto I'll breed my starter Pokemon and they'll all be in the WiFi Grand Trade System and you can just give me a Magikarp in return.. That's if you have WiFi.

Piplup is good later on because it has pretty good moves and at it's last evolution it is also a Steel type Pokemon. You can teach it a lot of HM's, Waterfall, Surf, Strength, Rock Climb, Rock Smash. Pretty much the same with Chimchar. On it's last evolution it is Fire + Fighting. It has a high Attack + Sp. Attack and it's fast so you'll be able to knock the other Pokemon out before they even get a hit. Turtwig however is a lot slower and has a high defence. It can only be taught Strength, Cut, Rock Smash and Rock Climb. Chimchar can be taught Strength, Rock Climb and Rock Smash.