Composing your letter: what you should never do.
Your leaving the company may be happening as a result of intolerable conflicts with someone; you may have come to the conclusion that you're deliberately being passed over all the time; in fact you may be mightily upset and spoiling for a fight.
Well, settle down! Sending in a letter that has to be written on asbestos may cleanse your attitude and let somebody know exactly what you think of them, but apart from that momentary satisfaction it'll likely do you more harm than good.
When you're angry and scribbling down exactly what you feel you're bound to be exaggerating some facts and distorting others: the result will be that when you have to account for what you've said you won't be able to. In a litigious society like the U.S. it doesn't take much for someone to conceive a lawsuit for libel (slander is spoken, libel is written) and it's no trouble at all to find a lawyer to take it on. And as Mattie Ross, the horrible little brat in "True Grit" pointed out: "Your lawyer will make money, my lawyer will make money, and I will make money, and you'll be paying us all", even if you win the case! Who needs that?
So leave it for a while before you compose your thunderbolt.
Once you're in a more objective mood, think carefully about what you're going to mention. Unpleasant remarks, even if they're wholly factual, will be resented. Personally-directed remarks will leave the recipient in a vengeful mood for a long time. There's the Sicilian saying "revenge is a dish best taken cold"; you may be running into the people that you're teeing off for the rest of your career and you can count on it that there will be some who'll take pleasure in ruining you whenever they can.
Remember that, if getting back at someone or messing up the company is what you want to do, your leaving is going to cause some problems for them. Best be satisfied with that: UNLESS you're (a) going to buy the state the company is in because you've won the lottery or (b) moving to Mars or (c) committing sepukku next week, and can therefore be sure that none of these people is going to be in a position to do anything to you.
(If you are leaving because you're in a whistle-blower situation, it's best to consult the state's employment standards agency to find out how to conduct yourself in the matter.)