Determining the "strongest" fantasy sword is subjective and often depends on the fictional universe or context in which the sword is portrayed. Different fantasy stories and settings have their own unique weapons with varying levels of power and abilities. However, here are a few notable examples of fantasy swords that are often considered exceptionally strong or powerful:

1. Excalibur (Arthurian Legends): Excalibur is the legendary sword of King Arthur, said to be one of the most powerful swords in mythology. It possesses magical properties and is often depicted as an unbreakable weapon of great strength and the symbol of rightful kingship.
2. Andúril (The Lord of the Rings): Andúril is the reforged sword of Aragorn, also known as the Flame of the West. It is described as a sword of kings, possessing a strong and radiant aura. Andúril is said to inspire courage and bring hope to its wielder.
3. The Sword of Truth (Sword of Truth series): The Sword of Truth, wielded by Richard Rahl, is a powerful magical weapon that can cut through almost anything, including magical barriers. It is known for its ability to defend against magic and evil forces.
4. Stormbringer (Elric of Melniboné): Stormbringer is a sentient black sword in Michael Moorcock's Elric of Melniboné series. It is an immensely powerful and deadly weapon that absorbs the souls of those it kills, granting its wielder enhanced strength and longevity.
5. Dragonbane (The Wheel of Time): Dragonbane is a sword wielded by the Dragon Reborn in Robert Jordan's Wheel of Time series. It is made of a special metal called cuendillar and is believed to be unbreakable. Dragonbane is imbued with magical properties and is highly effective against creatures of the Dark One.

These are just a few examples, and there are many other fantasy swords with unique abilities and strengths across various fictional worlds. The concept of strength can vary, encompassing factors such as magical properties, durability, enchantments, or symbolic significance within the story. Ultimately, the strength of a fantasy sword is defined by the imagination and storytelling of the author or creator.