The one you like best!
Any of them will do the job. Even the most primitive editors of 1971 will do the job. The most feature-bloated word processor of tomorrow will do the job. No editor or word processor affects in the slightest the "quality" of the text produced.
For PG purposes, therefore, the only difference between them all is how easy you find them to use, and what facilities they have for helping you--and those are decisions that only you can make.
If you already have a favorite editor or word processor, stick to it. If you don't, there's a huge selection available for you to consider, on any type of computer.
Sometimes, using a word processor, you may encounter some problems in saving your book as plain text. You have to figure out how to get it right just once, and then use that same method thereafter. If you have problems with this, ask other volunteers or one of the Posting Team for help.