I know everyones hating on grookey but I think his final evolutions really the best because cinderace is kinda girly and intelion is kinda creepy. also i think the drum idea is cute. and he has the best gigantimax form.
I've played with Scorbunny as a starter in Sword and with Grookey as a starter in Shield. I have a firm aversion to Water Starters, especially considering the unfairly favorable treatment Water Types have always seen in general, and the completely BS tradition of giving Water Type starters Ice coverage moves, negating their supposed "weakness" to grass types. That continues to be the case in this generation, and one could definitely argue Sobble and his line are mechanically the best options, considering the typing, coverage moves, and so forth. However that just makes Water type starters the worst, not the best.
For Scorbunny vs Grookey, I spent a lot of time in Sword with Scorbunny at the front. He did a lot of work, and I mean a LOT of work in terms of trainers, gym leaders, and so forth, and rarely needed someone else to take the helm. When he did need that, Corviknight was like the perfect partner, picking up any slack without issue. Grookey, conversely, was just not safe to have in front very often. He was too slow and had too many weaknesses, so he spent most of my playthrough sitting in the back, waiting for a time to shine that rarely came. I love the design. I love DK, played Donkey Konga, and Grookey is super cute. But realistically, he needs greater access to physical health recovery coverage moves (Draining Punch should be learned while leveling up) and needs either a stat or ability buff or Something to help overcome the weaknesses of Grass as a typing.
As such I gotta go with Scorbunny as the best starter. I respect those who like any of them, even if I personally strongly dislike Water Starters, but it's Scorbunny all the way for me. Grookey, I just wish he was better than he is.
While I love Scorbunny and I have a tendency of always going to the Fire type starter, I don't like Cinderace. In fact, the initial stage of these starters are great and very cute, but the final evolution was kinda disappointing.
All that said, as for starter, I choose Scorbunny. As the final stage, Inteleon.