Hi Tobbler,
Welcome to the forum, hopefully you'll also have a good time getting involved in discussions etc while you're here :)
To answer your question, when we first saw them my answer would've been Litten. However, it dawned on me that I've never used the Grass starter on my first play through of a game. It's only when I play back through the games at some point that I finally choose the Grass starter. Due to that, I'm actually gonna be choosing Rowlet this time around to see how it plays out. It'll make for a different dynamic for me :)