My son's skull. He had to have an inch-wide strip of his skull removed front to back and his forehead reconstructed with titanium due to a condition he was born with. That 5-day hospital stay was around $300,000 15 years ago, not to mention the multitude of specialist appointments, CT scans, xrays, etc. in the 6 weeks leading up to the surgery. Thank goodness for great insurance.
Other than that, probably engagement/wedding rings or my husband's honeybees. (I know that sounds dumb, but some people pay a lot of money for queens or nuc colonies).