He is amazingly voiced by talented voiceover artist Peter Cullen.
The toys based on him are great.
His design is creative, cool, and memorable, yet simple, and has been the basis of almost all of his other incarnations.
His death in the 1986 film was quite sad.
His appearance as a zombie in "Dark Awakening" is terrifying and disturbing.
He is very patient with others.
The rivalry between him and Megatron is extremely iconic.
The origin story of how he became Optimus is very cool, as he used to be called Orion Pax but after being crippled by a Decepticon attack led by Megatron, he was rebuilt into Optimus Prime by Alpha Trion.
The relationship between him and Elita-1 is very sweet.
His famous iconic lines such as "One shall stand, one shall fall." and "Till all are one."
While Prime's death in the 1986 movie was sad, it was really just done to blatantly replace him with Hot Rod/Rodimus Prime (even though he's not a bad character overall). However, he was eventually brought back in season 3 of the series after being killed off a second time.
At the time, Hasbro viewed Optimus as nothing more than a toy; they didn't think his death would really mean too much for kids, which was proven wrong because many were heartbroken in an unsatisfied way when Prime died, especially in a very horrid and disturbing way.
The death scene has also become very controversial for being extremely dark, cynical, morbid, horrific, and disturbing, even for a kids' movie. It's not like some kids' movies where the emotional moments are meant to be sad and emotional because Prime's death in the 1986 movie was literally made with disturbing imagery and scary music that can leave many feeling emotionally drained, depressed, and even traumatized.
He was originally going to fall apart after his death in the 1986 film.
In Japan, he's called Convoy as Japanese children would find it difficult to remember and pronounce his American name.
Him dying became a common joke in the Transformers fanbase.
Peter Cullen didn't know how famous Optimus Prime was until he saw the number of children who were leaving the cinema sobbing from being painfully traumatized by Optimus Prime's morbid death scene in the 1986 movie. Additionally, his agent showed him all the hate mail angry parents sent to Hasbro for doing something too serious, even for a kids' film.