I think Chesnaught has the best design out of its line. I disliked Chespin at first because I didn't like its design, but when I got a super cute plush of it, I warmed up to it. As for Quilladin, I think it looks a bit awkward, but I don't dislike it.
Regarding Grass starters, Venusaur is my favourite, followed closely by Sceptile and Serperior. Decidueye would probably be my fourth favourite Grass starter. After that, it gets more difficult for me.