Fennekin is my favorite. I love Fire, and the entire line looks amazing. Fennekin and Braixen are adorable, and Delphox is both adorable and bad-ass. So far, Fennekin to me seems to be the best of the 3 starters, both for in-game runs and in general.
I was initially indifferent to the Chespin line. I didn't hate them, but I didn't care much for them either. Now, I've come to really like the line. Chespin is adorable, Quilladin is goofy in a good way, and Chesnaught looks like a boss.
Oh yeah, and then there's the other one. I never really liked Froakie's line, and I think Greninja is in my opinion one of the ugliest things in Pokemon.