If you're talking about a server service, I can recommend Serv-U. We've used it for years. Very secure within it's core and you can add additional security as well. It's very easy to set up and use. Within 5 minutes--start to finish--you can have an FTP server up.
Robert Williams, Owner
Williams Web Solutions
www.williamswebsolutions.net
info@williamswebsolutions.net
Providing website design & redesign, website & e-mail hosting (reseller opportunities), server hosting, and computer IT services