Swords are a classic weapon in all media, not only anime, and for good reason. The many different styles and uses of the weapon, as well as the ways a sword can be personalized to the , make for exciting varieties in every show.
Some swords can only be wielded by one person, such as Samehada in Bleach. There are many qualities that can make a blade not only one of a kind, but one of the best of the best.
Claymores (Claymore)
Hordes of monstrous youma plague humanity, and the only ones capable of fighting them are a small force of warriors. They are known by the name of the weapons they wield: Claymores. The swords are the only things capable of cutting the flesh of youma, and are how Claymores survive in one of the scariest body horror anime.
The blades never dull or break, and it's implied that they're made of more than metal. They are almost as big as their wielders, and so heavy that Undine is famous for wielding two at once. When a Claymore dies, her sword is used as her grave marker, and will never rust.
Amol Saiqa (Magi: Labyrinth of Magic)
Metal Vessels are the most powerful items in the world of Magi: Labyrinth of Magic. When a warrior makes it through the trials of a Dungeon, a metal item of their choice is possessed by the Dungeon's Djinn, letting the Dungeon Capturer access their power by wielding it. Capturing Amon's Dungeon imbues Alibaba's dagger with his power.
When fully activated, Amon's blade more than doubles in size and turns jet-black: the better to absorb heat. It allows Alibaba to summon unlimited amounts of fire, both to attack and to create a defensive wall around himself. Its Extreme Magic summons an enormous swordsman made of fire that can break through anything, as seen in the episode "The Djinn Warriors."
Sekki (Noragami)
A god's Regalia is the most powerful weapon they can bring to bear in one of Studio Bones' best anime, whether they're fighting Phantoms or other gods. Delivery God Yato favors blades, and his latest Regalia Yukine is no exception. Yukine transforms into an odachi called Sekki.
Sekki's blade is incredibly sharp, to the point that in the episode "Your Voice Calls Out" he's able to cut through the Pacification Ring cast by Heaven itself. When he evolves into a Blessed Regalia (a feat many Regalia never achieve, but which Yukine reaches in less than a year), he transforms into two smaller swords, allowing Yato to duel wield him.
Kurikara (Blue Exorcist)
It's tough to beat a sword that summons the flames of Hell itself. Kurikara contains the demon heart of Rin Okumura, the son of Satan turned Exorcist. Rin's adoptive father Shiro initially gave him the blade to keep the power sealed, but he draws it anyway to try and save Shiro's life, freeing his formerly sealed demon half.
When drawn, Kurikara unleashes blue flames and reveals Rin's demonic nature. Rin must learn to control the flames in order to wield the sword properly, and as he does, he learns new techniques for fighting with them. In the episode "Shine Bright as the Sun," he successfully fights the Impure King without burning any of the friends fighting alongside him.
Samehada (Naruto)
Weapons handling is an essential skill for any shinobi, which means that the world of Naruto has no shortage of powerful swords. The best of all belong to the Seven Swordsmen of the Mist. Samehada, the weapon of former member Kisame Hoshigaki, is especially vicious. What's more, it's sentient and loyal enough to its wielder that it will attack anyone else who tries to touch it.
As Kisame explains, Samehada is like sharkskin: it doesn't slice, it shreds its victims to ribbons. It weakens its victims and strengthens itself by eating their chakra. Kisame can fight with it, hide inside it, and even fuse with it to become a humanoid shark creature in one of 's best moments featuring the Akatsuki.
Zanpakuto (Bleach)
Every Soul Reaper has a zanpakuto (roughly translated as "soul-cutting sword"). As one of the only weapons capable of cutting Hollows, the corrupted souls that threaten living humans, they are essential to their work. If the wielder is strong enough, they can unlock their zanpakuto's ultimate technique, or Bankai.
Because they are born of their wielders' souls, each zanpakuto is unique to the wielder and can grow stronger alongside them. For instance, Ichigo's Zangetsu evolves to the point of being able to fire explosive blasts of energy with wide range and incredible destructive power, making it one of the most powerful zanpakuto in Bleach.
Tessaiga (InuYasha)
Inuyasha and Sesshoumaru's father, the Great Dog Demon Touga, forged two swords from his own fang before his death. The more offensively powerful of the two, Tessaiga, he bequeathed to his younger son Inuyasha, much to the outrage of the older Sesshoumaru, who was left Tenseiga, the sword that healed instead of destroyed.
Tessaiga can slay a hundred demons in one stroke, and because it gets stronger the more it kills, it has several different powers for Inuyasha to master. These include firing beams of energy, reflecting enemies' attacks back at them, and even sending opponents straight to the Underworld as seen in the episode "Inside Naraku."
Excalibur (Soul Eater)
Soul Eater's Demon Weapons take many forms, including katana like Tsubaki and broadswords like Ragnarok. However, the most powerful in the whole show is Excalibur, legendary sword of heroes and kings and one of Takehito Koyasu's best characters. Excalibur's meisters can move at the speed of light, cut through dimensions, and fly on wings of light.
Unfortunately, Excalibur is also the least cool weapon in the show. As seen in the three filler episodes starring him, Excalibur can synchronize his soul wavelength with anyone, but the real test is putting up with his nonsensical rambling, list of 1,000 rules to obey, and egotistical and rude personality.
Saika (Durarara!!)
While Saika doesn't have as much destructive power as other swords, its power to control people is terrifying. Saika is a cursed blade that loves humanity and only wants to show that love, but its idea of doing so is cutting as many people as possible in order to possess and brainwash them. The hive mind under its control quickly spreads through Ikebukuro.
Saika's wielder, Anri Sonohara, must constantly resist the sword's attempts to possess her. In return, Saika grants her increased speed and reflexes, the ability to sprout blades from anywhere on her body, and control over anyone Saika has cut. Anri and Saika become a strong enough team to take on professional assassin Vorona in season 2.
Ea (Fate/Stay Night, Fate/Zero)
The power to rend Heaven and Earth themselves is more than enough for Ea to outclass all other swords in anime. It is one of the favorite Noble Phantasms of Gilgamesh, the only one capable of wielding it at all; the arrogant king will only bring it out against opponents he thinks are truly worthy.
Ea is older than any weapon in existence, in a world where the oldest magic is always the strongest. When Gilgamesh commands it to attack, it unleashes a great storm of winds powerful enough to tear through not just the Earth, but through space and time as well. Everything in Gilgamesh's sight is destroyed instantly with just one swing of Ea.