The one that sticks out to me the most is Froakie, because of its big eyes. Chespin is also very nice because it is the first mammal Grass-type starter (we had five Generations of reptiles so far). Fennekin looks very elegant, and because of this, it is very difficult to pick a favourite.
Fennekin looks the best of all, but I am more interested with using Chespin.
Thanks for reading.