When it comes to the Alola Starters..
Litten for Bulk
Rowlet for Speed
Popplio Overall
Get Rowlet from a WonderTrade or the GTS if you pick Popplio because you'll need Decidueye for Sun & Moon with Kukui because of Snorlax
I would ask why, but I know that I won't be able to understand. When I prefer one Pokemon to another I can rarely get why someone would go the other way.