Japanese swords, in general, are renowned for their strength and sharpness. The term "strongest" can mean different things, but often it refers to a combination of a blade's sharpness, hardness, and toughness. Here are some types of Japanese swords:

1. Katana: Perhaps the most famous Japanese sword, the katana is known for its distinct curved, single-edged blade, circular or squared guard, and the ability to accommodate two hands on the grip. It's crafted with a differential hardening process, leading to a harder edge (for sharpness) and a softer spine (for shock absorption), resulting in a strong and flexible weapon.
2. Tachi: This is an older style of sword that was typically longer and more curved than the katana. It was also worn differently, with the edge facing down as opposed to the katana's upward facing edge.
3. Nodachi/Odachi: These are very large two-handed swords used in various periods, often as ceremonial objects or used by infantry against cavalry. They're sometimes considered stronger due to their sheer size.
4. Wakizashi: A shorter companion sword to the katana, also made with high-quality steel and craftsmanship.

The strength of a sword is not only dependent on the type but also the craftsmanship involved. The technique of folding steel, differentially hardening the blade, and the quality of the steel used all contribute to the overall strength of the sword. Many consider the finest examples of katanas crafted by legendary swordsmiths like Masamune or Muramasa to be the "strongest" in terms of their quality, craftsmanship, and performance.

Remember, though, that "strength" in terms of swords is about more than just the ability to cut or not break. It also refers to the sword's balance, the precision with which it can be wielded, and how well it holds up over time with proper maintenance. Each type of Japanese sword had a specific purpose and usage, and what makes a sword "strong" can depend on what it's being used for.