In general but still about formats there are at least two reasons for saving things.

You may be most interested in protecting yourself from failures of equipment, which includes your own brain, and the goal is to allow recovery to a less recent version of what you're writing or perhaps something automatically saved by a helpful backup scheme. For that, the original format is surely the best.

But you might also be interested in what I call archiving. That means storage of the data in a form that can be read long after the next major change in the computing industry that might be three months from now.  The software that wrote those files no longer runs on any machine you now have. Nobody makes disk drives that can read those floppies which have decayed as a result of solar magnetic storms. The punched paper tape has become so brittle that it won't feed through the reader that I saved for the purpose. Nobody really knows how long home-written CDs will survive the elements.

For the latter, it seems that the only good format is graphemes chiseled into stone. Humans will probably be forever able to decode anything prepared that way. We now handle Inca and Sanskrit surprisingly well.

As for me I have Nisus files that had the wonderful property that the text was all in the data fork and formatting was in the resource fork. I have little trouble reading them today but, of course, Apple has pretty much made the resource fork unusable. I have long made it a point to convert my Excel financial files to tab separated text before archiving them. The formulas are not very useful after three years but the values can be important to my heirs.

RTF was introduced by Microsoft in the BASIC language which first ran on Macs. Nothing Microsoft or Apple makes can read that kind of RTF.  Even today Apple's RTF remains significantly different from Microsoft's and both change with each new OS version. The very meaning of the acronym has been destroyed by its use in email where it now is synonymous with HTML. I'd consider it a terrible choice for archiving.

So for my personal case plain old Latin text with ASCII encoding is clearly the choice. In a pinch formatting is pretty easy to restore though it would be possible for a restorer with bad things on his mind to change a few meanings.

Nisus does a good job of supporting non-Romance languages and archiving of that kind of text presents many more problems. Unicode is too new but at least it's public and isn't likely to get bogged down in secrets or changes made to demonstrate some corporation's superiority.

Public markup languages are worthy of thought. GML, general markup language, once had promise but it seems to have morphed into HTML which resulted in browser wars and possibly ruined HTML as an archiving possibility  for formatted text.

Adobe's portable document format, PDF, one had potential but I more and more find that my older machines can't read what's now called PDF. It has been improved which makes it pretty much useless as a candidate for archiving. Adobe postscript remains an potential option but it's not well supported for that purpose and it makes for large files.

Print and file. Used filing cabinets are cheap. Archival paper is available. Scanners and character recognition get better and better. CAD drawings are well supported on paper and nowhere else. It's a shame but it really may be the only solution for storage that will last for reading by your great grandchildren who will wonder why everyone didn't read and write directly into your brain with those attachments behind your ears.
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