S3. Sometimes I do that even when I'm transferring on my local network just because I know the flow so well. After all, my speed to the Internet is roughly the same as the speed on my LAN. They're both gigabit duplex.
Then I copy-paste that into a self-Slack and `wget` it on the other side.
It's almost equivalent ergonomically to a `python3 -m http.server` and because it's the same whether I'm giving to someone else or to myself, I spend less thinking!