MS Doc is totally obsolete and a crappy interchange format on top of that. If your reader can't do DocX, which is 13 years old, then you need a new reader.
Txt is also obsolete outside of a handful of specialized uses in the tech world. (It's the base format for tons and tons of log files and programming and scripting languages.)
PDF is "ok" at best, but isn't capable of reflow and the readers are universally a dumpster fire for security and more crash prone than a 1972 Ford Pinto with bad brakes. If you're on a tablet or computer screen it's probably fine for reading, but if you're reading on a phone or most ebook readers you're it's probably going to be too small.
Personally, I like ePub. It's designed to allow reflow for different size screens and devices, neigh universal outside of the Amazon ecosystem and trivial to convert to any other format.
This came up yesterday, but doc. files are my personal preference for back compatibility reasons. You can run them on most versions of Windows. However, I would publish in pdf. because it is more professional in my opinion. It also allows for easy conversion for most reading tablets.
I use .TXT because I can use it on any writing document program I've got. Plus, when uploading it to FA, people can automatically see the text of the story instead of having to download a file to open up.