Well, for your story, though the opinions of your readers does matter, this is YOUR story, you know? It's best to formulate the Pokemon that you want your character(s) to catch. Typically what works for me is that I choose Pokemon that I think would benefit for my character down the road. Each Pokemon, at some point, has their chance to shine are utilized in important plot elements. For example, if your character needs to get somewhere fast, it would probably be best to have a Pokemon that he or she can ride or surf on, or even fly with. It all depends on what you want because at the end of the day, it's your story. I hoped I helped you! (: