Not sure there's such a thing as the "best" medical phantom, but here goes another set: https://brainweb.bic.mni.mcgill.ca/anatomic_normal_20.html

Dear Dr. AlZu'bi

As you mentioned, the NEMA IEC body phantom is a popular one for imaging analysis. Moreover, the "Whole Body Phantom PBU-50" and "PBU-60" is a useful and well known phantom for CT imaging and 3D segmentation. its catalog can be found in the attachment. However, there some useful phantoms used in other imaging modilities(e.g. "IOUSFAN" for ultrasound imaging).

I hope it can be useful.

Regards.

have a look at https://grand-challenge.org/challenges/ hope will help.