What do you mean by secure? Secure as in vulnerabilities or secure as in the ease of the ability to lock the anonymous account down?
If you are talking about vulnerabilities, there aren't too many ftp servers that haven't been plagued by vulnerabilities for 2000. If nothing else, I'd probably go with Microsoft's if you are already subscribed to their bulletin list and are used to patching.
\"If you torture the data enough, it will confess.\" --Ronald Coase