DVD disk , Hard Drive or memory card- which is best for long term storage?

I have often heard that DVD or CD have a finite life but can anyone tell me what is the best medium for long term storage. Is a USB ext hard drive better than a DVD? Is a Re-writable DVD less dependable that a straight non re-write. How long before they lose what ever they lose? I am uming USB memory cards are not dependable for any long periods. thanks