It's an unspoken rule of parenting: you're not supposed to have a favourite child. You almost certainly have one, but you're not supposed to, on account of deep, complicated psychological stuff. Ask Freud.
Mercifully, Pokémon aren't children, and hence Niantic is keen to encourage Pokémon GO trainers to pick their favourite critter for a new 'Buddy' feature headed to their augmented reality app.
The next update headed to Android and iOS gadgets will implement 'Buddy Pokémon': trainers will be able to pick one of their collection to be their 'Buddy', earning Candy for that Pokémon as you walk a "certain distance", in addition to "unique in-game rewards and experiences".
It's not clear how far you're going to need to wander, nor whether different Pokémon will demand different distances to earn Candy (a Pidgey might only require 2km of walking for a Candy, while a Vaporeon might require a full 10km).
Unfortunately, your buddy won't walk behind you in a manner similar to Pikachu of Pokémon Yellow, but rather appear alongside your Trainer avatar on your profile screen. You'll be able to pick a new buddy whenever you want.
Update your app to get the new feature, and dust off your comfiest set of trainers. It's going to be a busy week of walking.