What authentication method is best:
The best authentication choice depends on your specific needs.  You can review the tutorial sections for each of the methods which list the advantages and disadvantages (basic, ntlm, cookie, ISAPI)

Generally if you wish to purchase a solution or have the experience to build it an ISAPI implementation provides the best flexibility and performance.

For internal or intranet sites where you know everyone to be using Internet Explorer, NTLM may be a good choice.

Cookie authentication is suitable for sites that only want to protect ASP scripts and have a lower load