I have used the 3 following FTP clients during the past years so here is my feedback:
WinSCP: the one that I currently use. There is a portable version, just one .exe file (and the .ini file for configuration). Don't forget to turn on the "Explorer" interface:
View > Preferences > Environment > Interface > Explorer, which is (IMHO) handier.
Benchmark: it is super fast! 4.5 sec to be connected to the server (time measured from double-click on the WinSCP icon until the server's file list is displayed).
Cyberduck: I've used it during years, it is great (double-click on a file => edit locally, CTRL+S in the local text editor => automatically reuploads without any popup, like WinSCP). The only drawback is that it is ... a bit slow. (Is it because it uses .NET framework and not pure WinAPI?).
Benchmark: on my i5 laptop, it takes 5 sec. for the UI to show (after double-clicking on the Cyberduck icon), and a total of 19 sec to be connected to the server (time measured from double-click on the Cyberduck icon until the server's file list is displayed).
FileZilla: a well-known FTP client, which once was probably the most popular one. I don't use it anymore because of this, which totally ruins its user experience, from a developer prospective.
ftp://server.com
into the location bar. (as an answer to this question, though, i recommend the already-suggested winscp.)ftp://username:[email protected]
It needs to be clear that it's Windows Explorer, not Internet Explorer, though.