Fast answer
Choose Cala Porta Vecchia when the beach should stay attached to the old town, Porto Ghiacciolo or Santo Stefano when the day should feel scenic, and Capitolo when sand, lido context, and a longer south-coast rhythm matter more than staying close to the center. Confirm services, access, parking, sea state, and crowd pressure close to travel.
If you only do one thing
For a first trip, do the easy swim first unless the beach is the main event: Cala Porta Vecchia for the lowest-friction swim, Porto Ghiacciolo or Santo Stefano for the cove day, and Capitolo for a planned beach-led day with public/private beach choices kept separate.