HDD is still the cheapest media. Placed in a drawer or cabinet it can last for many years. But: it's offline.
For online storage: do pay attention to the T&C (copyright) and the mode of presentation. If it's only for storing (without presentation) then there are more options than flickr.
There is no "best" option in general.
I'm pretty lay man n don't shoot alot thus i copy from the SD card to 2 HDD. When i'm ready to shoot again, i'll format the SD card.
If i'm in oversea on vacation where i will shoot more, i'll back up onto the WD wireless HDD.
you need to have a system for how you organize your images, this is far more important then where you store your images, it is pretty useless you able to store 10,000 images but can't find the pictures you were looking for, is just digital hoarding.
I agree with catchlight when he posted that organisation is important. For me, storage is part of your flow process. I use the following flow:
After shooting, image is transferred to an internal SSD in the computer (not C: / system drive). The images are sorted and tagged. Those images that I want to work on will remain on this disk.
All other images that were not deleted will be moved to an external Synology RAID drive. This is my working long term storage. If I need to access old images, this is where I go to. I can do searches since it is part of my lightroom catalog. I can also process the image without moving them back to the internal drive.
For archival storage, I will sync the image files in the Synology drive with a DLINK NAS, also RAID.The sync will update all the changes to the archive disk. The archive drive is my saftety backup and no edits are done that. At 6-12 months interval, I will burn the images to DVDs.
So far, this flow has been working for me. What I wanted was a quick access to images which I want to process and easy access to my old images. The hardware may change as better storage solution is available. What is important is that the archive can be access and read years (decades?) down the road.