Default as defined by port 80, no host headers and is running on the local machine:

private string GetDefaultWebPath()
        {
            string result = null;
            using (var de = new DirectoryEntry("IIS://Localhost/W3SVC"))
            {
                foreach (DirectoryEntry child in de.Children
                .Cast<DirectoryEntry>()
                .Where(d => d.SchemaClassName == "IIsWebServer" &&
                     d.Properties["ServerState"]
                     .Value.ToString() == "2" /* Started*/))
                {
                    var bindings = child.Properties
                        .Cast<PropertyValueCollection>()
                        .Where(i => i.PropertyName == "ServerBindings")
                        .ToList().Cast<PropertyValueCollection>();
                    if (bindings.Count() == 1 && bindings
                        .First().Value.ToString() == ":80:")
                    {
                        var root = child.Children
                            .Cast<DirectoryEntry>()
                            .FirstOrDefault(d => d.SchemaClassName == "IIsWebVirtualDir");
                        if (root != null)
                        {
                            result = root.Properties["Path"].Value.ToString();
                        }
                    }
                }
            }
            if (string.IsNullOrEmpty(result))
            {
                throw new Exception("Could not determine the default web.");
            }
            return result;
        }