An interesting subject, I'm sure that better, more efficient and more durable methods will evolve - eventually.
As many are aware, hard drives should not be relied on for long-term storage of important information as they can, and do, die. USB sticks can also become unreadable and/or lose data. Anything that uses electricity is susceptible to failure due to power surges and/or spikes, sometimes the simple act of turning them on can cause it.
Some years ago, before streaming services, CD/DVD burning was a big thing. There were forums dedicated to the subject with some very bright people on them. They did much testing of different discs to try and determine reliability and durability of different brands. Some were found to lose data (which you wouldn't expect on a burned disc, where the data is "permanently" marked on the media). Bottom line is that long-term retention of data on discs is very much an unknown, and anything considered important should be re-copied at regular intervals.
These days we often have large amounts of data to store, and while SSD's are still electronic, and thus subject to failure, they are fast and relatively cheap for their capacity. The best way is probably to store the data on the computer and then back it up REGULARLY to an external SSD.
I haven't mentioned using the cloud because I consider it the least reliable as I have no control over it. I don't use it for anything.
Crystal does sound good, or maybe storing data in the sub-molecular matrix of a piece of titanium?
