What is the hardest bolt to break?

I'm curious to know, what is the toughest bolt to break? Is it a matter of the material the bolt is made of, such as steel or titanium? Or does the size and shape of the bolt play a significant role in determining its strength? Are there any specific techniques or tools that are required to break a particularly strong bolt? Could you elaborate on the factors that contribute to a bolt's resistance to breaking, and provide some examples of bolts that are particularly challenging to break?