Sobble is my favorite so far. Chameleons are really interesting lizards and Sobble turning invisible when wet is a cool idea. Especially it secreting water or "crying" to turn invisible on command. I also think it has the most potential to go down an evolutionary path I will enjoy. However I would have wished for its head-to-body ratio to be significantly lower, and I hope we will see it climb or use its feet for something they are adapted to, because its grabby feet are really not made for walking or swimming.
Grookey is probably my second favorite, and I like that its proportions are more conservative compared to the other starters. I like the design but I'm not really on board with the drum/rhythm theme, and I don't really think the orange parts mix that well in with the rest of the color scheme. I hope it won't become too humanoid and/or profession-centric when it evolves furter. I know some people feel like we have too many monkeys and apes but I don' mind that at all.
Scorbunny is my least favorite of the bunch, but I still think it is alright, just a bit boring as far as a long anticipated Fire starter rabbit will go. I can't really see it going any other way than becoming a sleek and athletic humanoid rabbit as it evolves though, which is something I am not particularly interested in, but I might be wrong.