Bitcoin is arguably the hardest asset — fixed supply (21 million), decentralized, resistant to seizure, easily portable, and not reliant on any government or central bank. Over a 20-year span, it's a strong contender for capital protection, especially against fiat debasement.

Gold is the historical standard — scarce, durable, no counterparty risk — but harder to store, move, or verify.

Other considerations:

Productive land (with water rights)

Select high-quality equities (like Berkshire Hathaway or global monopolies)

U.S. Treasury Inflation-Protected Securities (TIPS), if you're betting on U.S. stability

But if you're asking hardest and most protective against systemic collapse, currency debasement, or confiscation — Bitcoin wins that debate.