Its open source & simple. Basically like FTP but its faster 'cause it uses multiple sessions simultaneously. Plus its free.

IS an FTP client!

Is the upload speed of your ISP ie your own connection. If it's fast, Filezilla is the best. If it's slow, Filezilla is still the best because it's reliable and will resume uploads after a shutdown.

VPN and remote type programs (ie Hamachi, TeamViewer) can be slow and unreliable. I also send very large files and Filezilla is my choice.

I've had good luck with bitvise SSH client, formerly known as Tunnelier. You can transfer files with SFTP, and it at least has the advantage that, if the connection is lost, the download or upload automatically resumes where it left off when the connection is re-established.

There's a server side SSH2 application there as well for download, which is also free for personal use. I've used both for years and find them easy to use and work with.

It's really the only alternative to FTP (or HTTP variations) if your upload speed is limited, or your client's download speed is limited.

If FTP isn't good enough, nothing else will be good enough.

I think SFTP is still a legitimate choice, so maybe you should add that back to your list of possible solutions.
[link: https://webrtc.org/|WebRTC] is a secure browser enabled transfer.
Rsync will provide file copies and check for existing data.
Innorix is a plugin for large file transfers
Aspera is an rsync alternative.

This list is ordered from free to not-so-free.