I love them all, really, but Grookey wins for me in the end. The whole line is such a fun concept.
Interstingly, I didn't start with Grookey, but Sobble (loved it and never started with a Water-type before). I was very lucky to get a Surprise Traded Grookey mid-game, so I got to play with two starters.
Scorbunny / Cinderace for me. Beyond having advantages vs the first gym in the game (when advantages are the most critical as your pool of pokemon is smallest), the fire type starter has the advantage of having the easiest time vs the virtually omnipresent steel and ice types in this generation. Not to mention having a defensive advantage vs Fairy types, having coverage moves effective vs Rock and Dark types, and being very fast and very strong, a great combo if you outlevel your opponent, which most people do during playthroughs in my experience. Plus he has the most powerful signature move, which helps.
Not to menion, bunnies are cute, and fire type starters have always been my preference going all the way back to Red, so it was really a no-contest from the beginning. The fact that I think Cinderace really is the best option is just icing on the cake for me, as I was never going to Not pick the fire type bunny starter.
Awesome "Avatar" picture (get it, it's a pun!) by =Qinni
I actually like all the starters but sobble is my favorite, he's so adorable!
When I first saw the evolution leaks I was a little disappointed because I didn't like drizzile that much but when my sobble evolved into it he was so cute!
Now he's an Inteleon and always shoves his face into the camera whenever I camp.