I always pick via final evolution because I know that that is the Pokemon that I will be using for the largest percentage of play time. Based on the lowest evolutions I actually think that Chespin is the cutest:
But I do love this picture of a group of Fennekins.
I holding out judgement on Froakie...
Now having evolutions on the table. These are my favorite fake evolutions:
I think if those were exactly what they ended up looking like I would have to chose Fennekin.
But a further point that I think should be made is the other half of picking a starter for me is the availability of in game catchable fire, grass, and water Pokemon. If there is a lack of good representation of any of those types and I'm more likely to pick the corresponding starter, assuming it's final evolution isn't ugly as all get out.