CD-R because it is still around after more than two decades and the media is still in production, there are many drives and they are repairable.
My second choice would be DVD-R for similar reasons, but less history.
Third choice would be FAT formatted spinning disks used as write once. But they are much more susceptible to environmental flux.
Anything that is expensive or hard to come by or new, I would avoid…they’re almost certainly going to be a Zip drive equivalent in 20 years because there is no consumer demand for physical storage and less and less commercial demand because of the cloud.
But that’s me so YMMV. Good luck.