There is no best format. If calibre can read it right now, calibre can read it in 150 years from now -- it's open source, so you can always build it yourself even after Kovid dies of old age
and Windows/Linux/OSX don't exist anymore
. The only long-term storage problems are going to be preserving the actual file (good backups in safe places) and having some program that can read it. DRM is the only cause of digital obsolescence, because any program that can read a file is not going to disappear. (MSWord is an excellent example of a program that is protected by DRM, hence it has obsolescence.)
calibre can easily convert between any format you have right now. There is no need to convert all your .mobis to EPUB just to keep them readable, because you can always convert them later. The fact that it is binary is completely irrelevant -- it doesn't stop the free open source programs from reading (and converting) it.
Keep the original format, since it has all the base information in it, and everything else can be derived from it as needed. Unless you have converted to EPUB for editing, in which case the EPUB obviously has more of what you need in it.
