I've read The Rule's of netiquette, and i think all the rules are important! but, If i have to choose one it would be rule #1 "remember the human". I've been using the internet for years, and i have came across a lot of rude e-mails and rude posts in chat rooms. I think a lot of people forget that they are talking to a real human with feelings. If everyone would remember this one rule treat a individual as you would want to be treated the internet would be a better place.
I don't believe there are any rules (no apostrophe) of netiquette. Instead, what is called netiquette is a set of rules.