What is the hardest asset in the world?

I'm curious, what is the hardest asset in the world? Is it something tangible like gold or diamonds, or is it something more intangible like intellectual property or brand recognition? Could it be a new form of digital asset, like cryptocurrency, that's difficult to understand and value? I'd love to hear your thoughts on what you consider to be the most challenging asset to acquire, hold, and trade in today's market.