It's tricky to give a single definitive answer to "hardest to break" because it depends on how you define "break" and what kind of forces you're applying.

Here are a few contenders, with explanations:

* Tungsten Carbide: This is incredibly hard, resisting scratching and abrasion better than almost any other material. However, it can be brittle, meaning it's strong under compression but weak under tension or impact.

* Titanium: Strong, lightweight, and resistant to corrosion, titanium is a fantastic choice for applications needing high strength-to-weight ratio. It's quite difficult to break but can deform under heavy loads.

* Steel Alloys: Many high-strength steel alloys like martensitic stainless steel are incredibly tough. They can withstand high stresses and impacts, making them ideal for structural applications.

Important Considerations:

* Ductility: Ductile materials can deform significantly before breaking.

* Toughness: This refers to the material's ability to absorb energy before breaking.

* Application: The "hardest to break" metal will vary depending on what forces you're applying (compression, tension, shear) and the environment (temperature, corrosion).

In conclusion: No single metal is the absolute "hardest to break". It's more about choosing the right metal for the right application and understanding its properties.