Here are thoughts and opinions of what I think of each starter Pokemon
Grass- He looks cool, but I feel he's just another Sceptile wanna be. Similar base stats, bad move pool, and similar strategies. Hopefully, he will he a Grass/Dragon type when he evolves into his stage 1/2 form. Most likely, the new grass Pokemon will be Grass/Poison due to the fact that it is a snake Pokemon.
Fire- I'm not to fond of his colors, I was hoping it would've been lava red, with some purple/pink mixture into it. Instead, we got black along with some yellow.... My guess on the base stats is the pig Pokemon will have high HP, well above defense/special defense, average attack/special attack, and he will probably have poor speed. Personally, I hope that the Pig will have the Fire/Psychic dual type combination, simply because they never made a combination like that, and Grumpig is a psychic type.
Water- I think the sea otter Pokemon has a lot of Potential to become a good OU Pokemon. If Game Freak give's the sea otter Pokemon good base stats, along with move pool. It looks really cute and I just couldn't resist it. I believe sea otter animals tend to be skinny and fast underwater, so it will probably be a really fast Pokemon with good special attack/attack, but poor defense/special defense. With the right move pool, this little guy (stage 2) could be unstoppable!