Given your other options for backup, HDD is the safest way to go. Other options include Magnetic Tape, SSD and optical media.
Let's examine the pitfalls of each:
More prone to erasure when exposed to a magnetic field than a HDD. Readers are also becoming harder and harder to find. You don't want to come back in 5 years and find that there's no way to remove the data from your medium.
Reliable in that there are no moving parts. They are prone to electrical degradation after several read/write cycles which is troublesome and potentially dangerous. The likelihood of losing data while the drive is not in use is slim, however.
The least reliable of the bunch. They're extremely prone to physical degradation (bending/warping) and it requires very little to throw them out of their deflection spec. Further, the encoding scheme used to write data to most optical media is rather complex, creating a greater likelihood of single element failure leading to unreadability.
Solid, sealed devices. Can be damaged by physical shock more easily than most of the above devices. Has precise mechanical parts that can lead to failed read/writes if damaged.
The benefit of HDDs, however, is that they ARE sealed. All of the moving parts are stored in an air-filtered enclosure. The magnetic stability of the bits on the disk is quite high and unlikely to change.
Further, if the mechanical parts fail, it is possible to have the platters removed and the data recovered from them directly.
There's no perfect option, but of the imperfect ones, HDD would probably be your best bet.