Hello,
The DICOM viewer that is probably the most used by radiologists is OsiriX and its fork Horos:
Both OsiriX and Horos are known to be compatible with Orthanc with respect to DICOM query/retrieve. The free version of OsiriX is limited (e.g. it will not display 3D images with more than 500 slices), but most of those limitations are lifted by Horos. Unfortunately, OsiriX/Horos is only available for Apple OS X. Other cross-platform, free and open-source viewers include:
I will not respond about proprietary DICOM viewers, as Orthanc is focused on the promotion of FOSS for medical imaging. You might find a list of other free viewers at the following address:
HTH,
Sébastien-