XPath: Get attribute by maximum value

xpath: /root//node[not(@y <= preceding-sibling::node/@y) and not(@y <=following-sibling::node/@y)]

XmlDocument doc = new XmlDocument();
doc.Load("data.xml");
XmlNodeList nodeList = doc.SelectNodes("/root/node[not(@score <= preceding-sibling::node/@score) and not(@score <=following-sibling::node/@score)]");
StringBuilder output = new StringBuilder();
            output.Append("<b>");
            for (int i = 0; i < nodeList.Count; i++)
            {
             output.Append("<b " +
                 nodeList[i].ParentNode.Attributes["parentAttribute1"].OuterXml + " " +
                  nodeList[i].ParentNode.Attributes["parentAttribute2"].OuterXml + ">");

                output.Append(nodeList[i].OuterXml);
                output.Append("</b>");
            }

            output.Append("</b>");