WinSCP matchmatches all your requirements.
Graphical: Yes ��� There are two alternative interfaces:
- Feels like a Windows explorer window: Yes – One of the interfaces is explicitly designed after Windows File Explorer.
- Supports drag and drop: Yes – Both between file panels and between other applications.
Uploading via drag&drop
Downloading via drag&drop - No nags/ads/spyware: Yes.
- Open source: Yes – GPL license
- Supports FTPS, SFTP: Yes
- All modern authentication mechanisms: Yes – SSH (for SFTP and SCP) is based on the up-to-date version of PuTTY. TLS/SSL (for FTPS, WebDAVS and Amazon S3) is based on up-to-date version of OpenSSL.
- Well-maintained: Yes – Over 1000 commits and 18 releases in the last year.
(I'm the author of WinSCP)