Bittorrent. No, really. Lots of nice behaviors when transferring large amounts of data between arbitrary endpoints.
Transferring the torrent metadata is pretty trivial and can be done via a wide range of methods, and having that flexibility can be nice.
Unlike HTTP, you get reasonable retry behavior on network hiccups. Also, more robust data integrity guarantees, though a manual hash test is probably a good idea either way.
At least among people I'm throwing TBs of data around with, torrent infra is common and it's nice to not have to deal with some special-purpose tool that, in practice, is probably a pain in the ass to get compatible versions deployed across a range of OSes. Basically every platform known to man can run a torrent client of some sort.
And obviously, no dependency on an intermediary. This is good if you're trying to avoid Google et al. That does, however, bring a potential con: if my side is fast and your side is slow, I'm seeing until you're done. If I'm uploading something to gdrive or whatever, I can disconnect one r the upload is done. If you control an intermediary like a seedbox, that's less of a problem.
In general, though, torrents are pretty great for this sort of thing. Just encrypt your stuff beforehand.
Sometimes I wonder if the bad connotation due to piracy made this protocol not standard (in a sense, that's not widely adopted, and people need dedicated clients, instead of it working for example in browsers).
I don't know enough, maybe it has security issues, or some limits... but the fact is that it's still widely used, by tech literate people, so it still stands the test of time and it must be doing something right.
WebTorrent [1] can be used to run BitTorrent on the web. It's actually integrated into the Brave browser which is a nice feature. Other browsers might catch on soon.
WebTorrent is pretty limited though:
> ... a browser-based WebTorrent client or "web peer" can only connect to other clients that support WebTorrent/WebRTC.
(from https://webtorrent.io/faq)
This essentially splinters the world into the bittorrent and webtorrent halves, which makes it far less useful.