Buy a fire-safe.
This is not just a good idea for storing backups, but a good idea period.
Exactly what media you put in it is the subject of other answers here.
But, from the perspective of recovering from a fire, having a washable medium is good. As long as the temperature doesn't get too high CDs and DVDs seem reasonably resilient, although I'd be concerned about smoke damage.
Ditto for hard-drives.
A flash drive does have the benefit that there are no moving parts to be damaged and you don't need to be concerned about the optical properties.
