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Within the Transformers franchise, Optimus Prime is one of the fiercest, wisest, strongest and bravest Autobots. Believing that freedom is the right of all sentient beings, he's valiantly fought against numerous Decepticon enemies. Nevertheless, there are those that even send a chill down his circuitry, with these being the most fearsome robots of them all.
Different Transformers continuities present varying versions of the same characters, and in some cases, variants of the Decepticons are powerful enough and terrible enough to make even Optimus Prime wary of their presence. This could be due to their power or their outright sadism, with these villains embodying everything he stands against. Of course, while his greatest and most terrifying enemy is theoretically not a Decepticon, he's still the ultimate evil against even a hero like Prime.
Megatron is Optimus Prime's Greatest Enemy
It might almost seem odd to classify Megatron as a Decepticon that Optimus Prime is afraid of, as the two have been battling it out consistently since 1984. Nevertheless, there are some versions of the Decepticon leader that far surpass Prime, and this is reflected in how they interact with each other.
Optimus Prime was truly shaken by the power that Megatron attained when he was exposed to dark energon in Transformers: Prime, remarking that he was now possibly unbeatable. He also had a very healthy respect for him in the anime Transformers: Armada, noting that he was perhaps the only Decepticon Warrior who could match him in a fight.
Transformers: Animated flipped their relationship entirely, with Optimus Prime being a young and completely inexperienced Autobot who was well aware of the legendary terror of Megatron. In his team's first encounter with the Decepticon leader, it took all of them to take him down, and even this defeat, while grievous, proved to be temporary. Thus, Megatron can definitely strike fear into the sparks of various Autobots, including Optimus Prime himself.
Shockwave was the Most Logical Threat to the Autobots
In the original Transformers cartoon, Shockwave was merely a toadie for Megatron and was never seen beyond Cybertron. This wasn't the case in the Marvel comic book continuity, wherein he was as treacherous as Starscream while being far more successful in these endeavors. In fact, this saw both Megatron and Optimus Prime suffer horrific defeats, with Shockwave decimating Megatron in combat to take control of the Decepticons while single-handedly defeating the Autobots.
This left Optimus Prime as a disembodied head, rendering him completely helpless. Other continuities have Prime remarking in terror at Shockwave's machinations, with the Aligned Continuity of Transformers: Prime having Shockwave as the greatest military and scientific mind of the Decepticons. Likewise, the IDW Publishing comic books saw Prime realizing how thoroughly Cybertron's very history had been manipulated by Shockwave, completely shocking the Autobot leader and making the Decepticon live up to his name.
Prime Believed His Fight Against the Fallen Could Be His Last
In most Transformers continuities, The Fallen is an ancient Transformer and one of the 13 original members of the species. Corrupted by Unicron in the past, his appearance is ironically similar to Optimus Prime, namely, his head design and faceplate. Thus, modern timelines have cemented him as one of the original 13 Primes, with his true name being Megatronus Prime. While these incarnations usually aren't well-known by their descendants, one version was rightfully feared by him.
The version of The Fallen in the live-action Transformers movies had a very different design and somewhat different backstory than usual. Nevertheless, he possessed abilities far beyond those of normal Transformers, and even powerful Decepticons such as Megatron, with these being almost cosmic in nature. When he went off to fight him, Optimus Prime knew he might not come back. Despite this, his valor won out, allowing him to defeat the ancient evil.
Nemesis Prime Is Optimus' Dark Mirror
The concept of Nemesis Prime debuted in the anime Transformers: Armada, wherein the creature was a Unicron-controlled copy of Optimus Prime in darker colors. This idea has carried over to other continuities, ensuring that the Autobot leader always has a black repaint. The idea of a mirror image to his own heroism and what he stands for is something that Optimus clearly disliked, but it's one version of Nemesis in particular that struck a particular chord with him.
The Aligned Continuity version of Nemesis Prime was actually created by humans as part of the organization M.E.C.H., with this monstrosity actually matching the real Autobots' strength. This alone didn't make him fear his imperfect copycat, but it was the implications of what Nemesis Prime could do to the Autobots' good name.
Optimus was more than aware of how Nemesis Prime was being used to tarnish the reputation of Transformers as a whole, as well as the general level of damage, destruction and mayhem in its wake. Thus, he presented the type of threat that even the likes of Megatron didn't. Conversely, one version of the concept was a truly corrupted version of Optimus Prime himself, with this lack of inhibitions and his own heroic spark being something truly fearful for the Autobot.
Unicron is the Ultimate Enemy of the Transformers
Though he's an enemy of all Cybertronian, Unicron's evil makes him far closer to a Decepticon than an Autobot. The Chaos Bringer is also the brother of Primus, the Transformers' creator god and the true form of Cybertron itself. Optimus Prime has openly stated in different continuities that Unicron is far beyond anything the Autobots can defeat on their own. Ironically, the Autobot Matrix of Leadership that Prime hosts is usually the one true threat to the planet-eating robot.
Unicron debuted in The Transformers: The Movie, with Optimus Prime dying before the enemy ever even encounters the Autobots. Later timelines changed this to have Prime well aware of Unicron's threat, with Optimus clearly filled with dread over him. Interesting enough, this trait was also seen in G1 Optimus Prime's Maximal descendant, Optimus Primal. In the first season finale of the sequel series Beast Wars: Transformers, Primal is tortured by the alien Vok when they use the visage of Unicron, showcasing how his devilish evil was still known and feared.
