In order of how often I see families wish they had it:
A durable power of attorney for finances. Without it, helping with a bank account can mean a guardianship proceeding — expensive, slow, and public.
A health care proxy naming one person. Not three. One, with a named backup. Committees do not work at 3am.
A current list of accounts and where documents live. Not the passwords, just the map. I have watched families spend months finding what took an afternoon to write down.
None of these are about death particularly. They are about the far more common situation where someone is alive and needs help.