In nearly every version of the show and comics, it seems, Optimus dies at least once. I suppose it has become almost a badge of honor for the show, "Oh, we've made it to the point where we can kill off Optimus!" Still, there are plenty of fans young and old who have been and always will be traumatized by their hero dying. I decided to write this as if Optimus were speaking to the fans right before his death.
Gah! The feels. This will hurt to write.
Optimus Prime was dying. He was not afraid, nor even disturbed. Death was a familiar presence to him now. Still, it got harder every time. The first few deaths had confused him slightly, but he had accepted the cycle and moved on, thinking nothing of it. After a few more, however, he began to notice Them. They were just wisps of sound and image at first: a man's face, full of sorrow and betrayal. A little girl tugging on her babysitter's arm and whispering, "Why isn't he getting back up?" A woman hunched over a mobile screen, moaning, "Not again, please not again!" He tried to ignore Them at first, but as time passed, they became as real to him as his own form.
He saw them now, staring at screens, or pages, or empty space. His own people stood behind him, anguish written clearly upon their faceplates, with some of their human allies openly weeping. He knew they were there, yet his attention was focused on Them, the Ones-who-were-there-and-not-there.
His dimming optics softened. "Please, young ones, do not weep. This is how it must be, how it always must be." I thought this time would be different! An angry looking young man sitting in a stairwell held his gaze a moment. Optimus shook his helm. "No, Stephen. If given the choice, I will always put you and the others before my own well-being. You know that." Why are you dying? You can't die! A little boy stared in dismay at his hero. "Kyoungil, you will understand someday."
Optimus saw others, those who had stood through this cycle many times before. They stood with jaws clenched, unwilling to shed tears, trying to be brave for his sake. These were the ones who had looked to him for guidance during their childhoods. "Please, do not weep," he pleaded, "You know this is not the end." A man held his young daughter up on his shoulder. Say goodbye to Optimus, Carrie. She looked confused, hurt. But he's got to fight the bad guys! She protested. "Carrie, the war is over now. You don't need me to protect you anymore."
But what if the bad guys come back? "Then I will return, Patil." Do you promise? "Yes, I promise." Optimus saw a young woman who bit her lip and clutched a worn metal truck to her chest. "How many cycles have you seen, Christine?" he asked kindly. F-four, sir. "Then why do you look so sad?" This one seems so final, sir. "They always do, child. Surely you know that I will always return?" A young man in old clothes wiped his eyes. It won't be you! Optimus smiled at him. "Aidan, you said that the last time the cycle occurred. Do you remember what you said when I came back?" The young man smiled through his tears. I said, "hello, Old Friend." "Exactly." The Prime rumbled.
The Ones-who-were-there-and-not-there began to calm themselves, with the more experienced moving to comfort the ones who had never seen him die before. This was always the part that hurt most. Optimus tried to smile at them, but there were too many faces and too many tears to make light of the situation. "My friends," he began, "Let me tell you something an old friend of mine once told his granddaughter. During all the years I've been taking care of you, you in return have been taking care of day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine."
The mighty Prime's optics dimmed, and flickered out. His armor gently dropped to the ground, and his allies behind him cried out his name, as did many of Them. Those who had been standing near enough to hear his last words began to pass them along to those standing further away. It had not been "goodbye". It had never been goodbye, and never would be.
His last word was, "Hello."
Bold Print taken from Doctor Who: The Dalek Invasion of Earth