I work at a movie production company and we have a lot of film post production and release happening. All types of formats are handed to us by filmmakers and other studios. Now we want to archive all the files to a storage device and start moving them to LTO devices. What do you think is the best file format for storing the films? Note that the files are supposed to be ready to be accessed later for the purpose of preparing for screening and subtitling.
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.
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.
Ah yes, the age-old question without a definitive answer that is straightforward and cost-effective.
In short, you want to store them in the highest-quality you can, in a format that you'll hopefully not have trouble opening in a few years.
Some of this depends on the quality of your source material and your workflow. Archivists will often tell you to use uncompressed 10-bit video streams, or something like TIFF or PNG sequences. This, of course, can send the size of your project north of 1TB/hour, is not terribly practical, and not at all necessary for most projects out there.
I should first provide the disclaimer that the following recommendations are common practice but not technically true archival formats. But it is my opinion that it doesn't really matter.
If you're a Mac-based facility (or can export the masters from a Mac) then ProRes is still probably the best balance. If you're working with very high-quality source material with a lot of VFX/color information, ProRes 4444 is available, but honestly most people would be find with ProRes 422 HQ.
If you're on Windows/Premiere, there are other options. I'm personally a can of CineForm, which began as GoPro's codec but has since been open-sourced. It has 10-bit 4:2:2 and 12-bit 4:4:4 modes with output quality on par with ProRes but, in my experience, is a bit less resource-intensive to decode. It's also not ubiquitous -- not all software can decode CineForm, so beware of that, but as it is open source, it should not be difficult to find a tool that can handle it. (Premiere and FFmpeg both handle it fine.)
As an additional option, and what the movie studios tend to use (out of habit), is Avid's DNxHD/DNxHR codecs. Similar settings and output as the previously-mentioned choices, though it is arguably inferior in a few ways and has some licensing caveats.
One thing that negates the fears about these not-quite-archival-caliber formats is to also just archive the video project itself with all raw footage, project files, assets, etc. self-contained. But this is expensive and down the road someone will need to open up the project and export something, so, not always ideal.
I tend to use CineForm and occasionally ProRes HQ, and hold onto the raw footage and project files when possible. You'll have to play around with options and decide what is best for your use-case.