Horos/Osirix is - by far - the best open-source DICOM viewer, it's the only viewer I'd describe as being on-par with commercial PACS software. Unfortunately it's macOS only.

You can use InVesalius https://github.com/invesalius/invesalius3

slicer.org is a great open source viewer, the learning curve is something but the results can be impressive.