My favorite is Fennekin by a mile at this point, I just love its design so much and Fire is my favorite type out of the three starters. Froakie is a solid choice too, but Fennekin has the edge for me. I'm underwhelmed by Chespin, I don't really like the design (I didn't like Dewott either) and Grass is my least favorite out of the three types.
See and I think Gogoat is the best out of all of them. I like Helioptile the least out of all of them but I think it has potential to evolve into something really cool so fingers crossed. We'll find out more on the 19th hopefully.