Basically, here you’ll want to prioritize Pokémon for which you don’t already have the max evolution…

Basically, here you’ll want to prioritize Pokémon for which you don’t already have the max evolution and that only take 1 km to get a candy. For me, that means Pikachu, Jigglypuff, Geodude and Magikarp. I’ve gone with Pikachu first, both because I’m fairly close to a Raichu and because I have a notion to reenact the anime. If you’ve already got the max evolution for all the 1 km Pokémon, move onto the 3 km, and so forth. Once you’re up to 5 km Pokémon it becomes a bit of a grind, but that’s the endgame we’re talking about.

For a full list of how long you’ll have to walk to collect candy for each Pokemon, click here.
https://pokeassistant.com/buddystats?locale=en
or here
http://www.pokemongochamp.com/buddy-system-chart-candy-distance-size/