Well, that depends on what you are talking about. The flash cards, or hard drives for long-term high-capacity storage?
For the latter, the best storage device is any hard drive that you back up to an external drive. Consumer HDDs are commodities these days; the difference is whether you buy a fast one or a quiet, energy-sipping one. _No_ hard drive is reliable enough for storage of anything that you will need a year from now; that's why the good Lord gave us backup software.
the best storage device for your in-camera storage would be a high capacity memory card. a big, roomy hdd would be needed to store the photos when the memory card fills up. you'll need multiple high capacity, high write speed memory cards so that your camera stores the photos as soon as you take them and get ready to take the next photo.
you should choose memory cards that your camera supports, check which type of memory cards your camera uses (sdhc, mmc, xd, ms pro,compactflash etc).
I thank both of you guys for your information, but there has to be some off-line storage devices out there which can be used for long term storage other than flash drives and memory cards. The other problem with these devices is you cannot easily organize your pictures. I have DVDs, flash drives and memory cards, I just purchased a 32GB and 64GB
memory card, lying around every where. Plus, I have thousands of pictures in storage from when I shot film. I need to get this sturr organized in some off line stroage.
A hard drive _can_ be offline storage, if it's removable. You are asking for the best way to store and organize a large amount of data. That's a hard drive or, if you want to spend a lot of money, an SSD. If you really want that storage to be offline, use a removable hard drive.
Or get a cloud storage account. Myself, I would never trust a cloud provider to keep my data intact, but it is offline, large, and easy to organize.
For bulk data storage it's hard to beat external hard drives for speed, capacity and cost. But be aware that ANY storage repository can fail on you. It doesn't matter whether it's a hard drive, an optical disk, flash memory, or even a cloud storage provider (companies can and do go out of business without warning). So you need to have at least two copies on separate media, and ideally stored in separate locations.
As sminlal indicated, USE two storage devices. IE two External HDDs, or a External HDD and a DVD drive. Blu-ray has higher capacity, but also higher cost per disk. For cost a Blu-ray SL is about 22 gigs vs 4.7 Gigs for DVD SL.
For External HDDs. Understand, they do fail. Also for long term storage the "Cells" (magnetic domains) can corrupt adjacent Cells when stored for long period of time also the bearings can create a problem when stored over a long period. Solution is to periodically power on and rewrite the data.
For DVD disk.
(A) DO NOT use permant markers to write on. The acid in them can eat thru to the recording media - Bye-Bye Disk, they do sell a marking pen safe for DVDs. Not a fan of "Labels" but have used my ink-jet to print on printable version.
(B) Store in Cool DRY location. High relative humidy decreases how long the disk will retain the data.
(C) Do not write above the rated speed, I often write to the disk at 1/2 rated speed for disk I plan on keeping longer than 5 years.
(D) Use single layer disk, not the dual layer (DL) ones.
(E) Buy QUALITY Dvds, not the cheapest one you can find. Most writers will so a preference for a given brand/lot. I normally runs a program to measure the quality of the burn on two samples from the package.