What is the most difficult burden of proof?

Can you elaborate on what you mean by "the most difficult burden of proof"? Are you referring to a specific legal context, such as criminal or civil proceedings? Or perhaps you're referring to a broader concept, such as the challenge of proving a certain fact or theory in the realm of cryptocurrency and finance? If we're focusing on the latter, then it's worth noting that the burden of proof in cryptocurrency and finance can be significant, given the complex and often decentralized nature of these systems. For instance, proving ownership of digital assets or tracing the Flow of funds through a blockchain can be difficult, as these transactions are often pseudonymous and lack traditional identifiers like bank account numbers or names. Furthermore, proving intent or knowledge in cases involving fraud or money laundering can be challenging, as the digital trail may be difficult to follow or may be obscured by encryption or other security measures. Ultimately, the most difficult burden of proof will depend on the specific circumstances of the case and the level of evidence required to satisfy the relevant legal standard.