What is the best starter Pokemon to choose?

There is no specific best starter. I'll just give you my opinion on each of them.

Turtwig: Turtwig is the Grass Starter. Turtwig is capable of learning Grass moves a bit early, giving it a quick advantage. Turtwig evolves into Grotle at Level 18, who is capable of learning Leech Seed. Grotle then evolves into Torterra at Level 32. Torterra becomes part Ground, making it capable of unleashing the great move, Earthquake. The problem is that it's quite slow and gets killed by Ice Beam easily.

Chimchar: Chimchar is my personal favorite choice. Chimchar, like Torchic will have trouble with the first gym unless you evolve into into Moneferno at Level 14. Monferno can learn Typhlosion's move, Flame Wheel. Monferno is also like Combusken, it's part Fighting. Monferno then evolves into Infernape at Level 36. Just like Blaziken, it's Fire/Fighting. It learns a great deal of Physical attacks and has the attack stat to use them, he's quite speedy too.

Piplup: Piplup is the water starter. Piplup doesn't learn any good moves early in the game until Bubblebeam, giving it a hard time. It evolves into Priplup at Level 16. They still aren't anything special though. At Level 36, Prinplip evolves into Empoleon. Empoleon is part Steel, giving it more some good defensive parts now.

This is just from my point of view.

We don't know...since the game is not out yet we haven't seen the starters potential...on the contrary...i would choose either the Grass type or the Fire type...

Pick chimchar because you see that in the final stages Infernape(chimchar's final form) is Fire fighting turtwig will become grass/ground piplup will be water/steel.

Piplup

I personally like Piplup, It is my best Pokemon and is really good in battles. If you choose this one then make sure it learns surf and cut because those are really effective in battle.