Personally I tend to stick with Pokémon that I want to evolve soon. I like to choose the cuter ones when possible (recently I was walking a Phanpy for ages and my god it was adorable ♥) but right now I'm walking a gross Snubbul just to try and get evolving him out of the way so I'll ever have to see him again. Using all my Rare Candy on him too - don't want to have to look at him for much longer!
I usually trake starters along with me, if they are close to their final evolution. They're rare enough and take so much candy to evolve. Also it makes me feel like a real Pokémon Trainer with her starter <3
I really love this mentality I wish I could share it!