The most important editor to know is vi. The reason is simple - if your system is trashed emacs might not work but vi is always going to be available (at least it will if your system can boot enough to give you a command line). When you've got a dead system from some /etc/ file being corrupted or whatever is not the time to find out emacs won't edit your file it since you can't start the program. So at least learn enough vi to get you by. If you find you don't like it much then learning other editors is great too. But do not completely ignore vi . . . .