In which text editor is it easiest to prevent and / or remove watermarks?
It is impossible or almost impossible unless you knows the right text. The watermark is generally a marker that can later be used to distinguish a single copy. This can be as simple as having two spaces in a text. Edit this answer and you will see that.
Other options may be replacing some words. For example, I can substitute "I had noodles for lunch today" with "I had fish for lunch today," and without knowing whether the text is in any other form, you have No Kind of telling. For me as an author, it would be trivial to find out who leaked if each recipient had a clear text, but no recipient could detect it without talking to other recipients.
Another good alternative would be to write a word here and there wrong. This may look inadvertent to the recipient …
That said. If you want to exchange text, swap text. No word files or other obscure, possibly binary formats. Word files can even contain active content such as Visual Basic macros!
If formatting is desired, I would suggest open, verifiable formatting, e.g. Markdown or HTML.