Thinking of a note always reminds me of this early Grey's Anatomy episode. It was about a terminal patient who asked his doctor to tape his goodbyes to all the people he wants to say farewell to. She finds it really sweet and starts the camera but then the dude starts cursing all the people who had done him wrong in the past in the most awful ways. After a while, the doctor stops the tape and asks if that's really how he wants to be remembered and he says that all his life, all these people overlooked him, did not consider how their actions hurt him and no one ever knew how he truly felt. This was his chance to finally be heard.
When someone dies, a lot of people wish they could have told them something they never did or ponder their last words to the deceased. With suicide in particular a lot of the people you leave behind will wonder about these things and your last words to them as well. My advice would be what I have been advised by the lovely GoodPersonEffed the other day: write from the heart. Write what you want to say, what you have always wanted to say but never dared to. Write to comfort those who you think will be hit hardest by your death. Those who you worry will be the most devastated about it. But most importantly: write the truth.
Those who ctb get to choose their last words to any- and everyone, many don't get that luxury. What you write is up to you; it can be short, long, apologetic or angry. Sad or happy, reassuring or accusing. It's your choice. Just be sure that what you write is what you want people to remember as your last words as they will reverberate long after the final bell has been struck.