To get element with the highest ID using XSL you can use the following snippet.
XML:
<rootnode> <node> <id>3</id> </node> <node> <id>2</id> </node> <node> <id>1</id> </node> <node> <id>5</id> </node> <node> <id>4</id> </node> </rootnode>
XSL:
<xsl:variable name="MaxNodeId"><xsl:value-of select="/rootnode/node[not(/rootnode/node/id > id)]/id"/></xsl:variable>