DoomCAD Windows based. Shareware, but is completely uncrippled. I'm not sure about the others, but the latest version of Deu is 5.3. These editors are available from the various Doom ftp sites, ie. In here you will find 5 relevant directories, as well as other useful directories, all containing many useful programs, editors and suchlike.
Some editors are harder to use than others, some have more facilities than others, and so on.
I recommend just trying some of them out to see which one you find easiest to use. Just try them out. Some people use more than one editor in order to 'get the best of both worlds', as it were. All the editors come with help documents on how to get going, some more so than others. Deu now has quite a lot of help stuff written for it.