As someone pointed out, it depends on the level of threat. But one that will be useful to you anytime you need encryption and anonymity is onionshare service.
Onionshare ( Github respository ) ( Official website )
Its anonymous and encrypted file sharing service that runs on tor network. It's also open source and end to end encrypted.
Now what's great about this is that the files you want to share never leaves your system, instead onionshare starts a temporary web server on your system which contains only those files you want to share and makes it available as onion service and provides you with a unique URL that is accessible only through tor browser.
It simply means you are hosting your files on your own system that can be accessed through tor network with a unique and unguessable url. Another major feature is the automatically shutting down option of the server when the files are downloaded. It's like it never does exists.
There's also another security feature in onion share called stealth mode. When activated it generates another string along with url called "HidServAuth ..." which the recieving party has to set inside the tor browser config to access the url. This works like a password.
What's an Onion service ?
It's just a name. Technically its a service hosted on a server hidden inside the tor network. As an onion have layers to peel, so does the tor network ( layers of encryption ), hence the name. You can read more about this on official site
The only weakness in all this is how you share that URL. PGP is of course the recommended way but you can also use end to end encryption services instead of ISP cellular ( they can easily snoop ). Another weakness that you may find is the exit node, but that doesn't apply here as your temporary server is inside the tor network.
Use stealth mode. You will have two vectors to mitigate risks ( URL and Auth string ). Share it through different mediums.
For anonymity you can use the hardened operating systems available. Search on google.
PGP is still everywhere.
- And you can also use stegnography for hiding them in plain sights. You can use Openpuff for that.
Onionshare is available as gui on windows, linux and Mac OS on the official website.
If you want custom build, you can follow the wiki
Signal for end to end encryption
The one that you can use is Signal developed by Open Whisper Systems. These are the guys that designed the signal (end to end encryption) protocol for mobile systems. They got some high profile endorsers. Even whatsapp uses it for end to end cryptography. The major benefit is that its open source. Read more about on wiki.
Signal is available on Android, IOS and Desktop.
Signal Official website
Signal github repository
And of course you don't know exactly how any of the end to end providers servers are implemented and what they are retaining, so there's that....
Now the anonymity aspect of tor depends upon the underlying Operating systems and the network configurations and several other aspects but the privacy is still there. So, i would recommend onion share.