No help with "best".
For most expedient, I would heat the remaining ring of PVC in the fitting until it softened. Then use a flat-blade screwdriver to push the softened plastic toward the center. Then glue a new fitting in, and don't stress it this time. There are also stronger adapters to pvc including "schedule 80" pvc and you could adapt to metal threads.
For heat, a heat gun would be good. Some hairdryers may be enough, but I doubt it. A propane torch moved around, and largely heating the rusty metal around the PVC would probably be good.
I am not a plumber. I have not actually tried what I described.
If I had to name it, I would call it some kind of special flange.