What is the best format for archiving digital movies?
If you're looking for an open source codec that is finding favour in archives and museums as an archival format, you might want to look at FFV1 from the FFMPEG project.

https://en.wikipedia.org/wiki/FFV1

It is reasonably fast, reasonably efficient, cross-platform (including linux), free, and supports up to 16-bit per channel colour. Storage size is similar to ProRes.

However the main advantage is that it is 100% open source and non-proprietary, so it's not going to suddenly disappear come the Patent Apocalypse. It comes with ffmpeg, which is hands-down the best tool for doing these kind of conversions anyway.