This question is always a tough one, and to find the best fit you really need to drill down to what you want to experience there. Every place has its pros and cons. But if it puts your mind at ease, I feel like it’s a little bit splitting hairs.
Are you interested in exploring Naples and/or Pompeii? Then Sorrento is your ideal choice. It’s a very pleasant place with superb transport connections. However, I would not go out of my way to see Sorrento itself. Day tripping to Pompeii from AC is not ideal.
Are you mostly interested in relaxing in one place with a side trip or two? Then I’d say Positano. It’s definitely the most impactful, romantic of the choices. A smaller town like Priano/Maori would be good too, if you’re not planning on doing a whole lot.
Are you interested in exploring more of the Amalfi Coast (Ravello+smaller towns)? Then Amalfi might be the pick. It’s not as pretty as Positano, but is THE transport hub on the AC. You can get ferries everywhere here, and there is a big bus station that makes going to places like Ravello a snap (highly recommend Ravello.)
Capri is accessible from any of these places, and I think is fine as a day trip. Take the earliest ferry you can, and you’ll be hours ahead of most tourist groups. There were zero crowds in AnaCapri until lunchtime and the island was teeming.