What is the best starter Pokemon?

Unlike the regular series, It really doesn't matter, you can catch/hatch all of them throughout the game.

True starmie but it is quite rare to find one that is extra large in both weight and height

Short answer pikachu, but people often say it does not matter and they don't really tell new players why so I will break it down.

1. The starter pokemon are rare in most areas. Even if you love your starter pokemon it is an unrealistic task to try and amass enough candy to keep it leveled up with you.
2. If the game has been out over a week in your country then you won't be doing a gym battle until your trainer is in the mid to high teens with pokemon of CP 700+. The only thing you will live for is catching and evolving pokemon till you hit this barrier of entry.
3. With the exception of pikachu none of the other starter pokemon are the best in their class. Eevee is in fact the most powerful pokemon in that regard because 2 of its evolution are best in class.
4. Pikachu is the default "best" starter because you get 3 candy for picking him and a 4th when you send him off to the grinder. Those candies can be used to level up the pikachu or raichu you find at later levels.

What StaticRadion said is mostly true. What defending pokemon you have matters rather little, so it becomes all about the DPS. Of the four starters, Raichu was the only one with the highest DPS for its type, but the list has since been corrected to also include Venusaur. Their DPS make them the best pokemon to strive for of all four fully evolved versions. What Bulbasaur and the non-Pikachu starters have over Pikachu however, is candy value.

Pikachu requires 50 candies to evolve into a Raichu. Once you have your Raichu, the rest is just for XP.

In contrast, the other three starters require 25 candies to evolve once and 100 to evolve the second time. If you want to fill out the pokedex, there's a greater need for candies of the three starters. Of course, whether you do or don't, you'll eventually only be using candies for XP. Since all evolutions give the same XP, the candies for cheaper evolutions have the most value.

If you could choose a Pidgey as a starter, it would probably be better than all the others simply because of the extra XP you derive from their candies. As StaticRadion said, your starter is never used, and thus only exists to give you candy. If you really want a Raichu as fast as possible, go ahead, (it doesn't make much difference after all) but as soon as you have a Raichu, additional candies could have better gone towards more efficient XP gaining.

It's only four candies though. Pick whoever you want in order to be able to say to your friends "I picked _____"

I might be a little late, but I would say Bulbasaur.