This may be a better solution.  Rather than having multiple variables for 
each attribute it is probably better to store the Items node that has the 
maximum atrr1 value then the variable holding the node can be queried for the 
other atrr values as and when needed.

Also it's better to move the code to retrieve the maximum atrr1 value to 
it's own template reducing the clutter in the main template.

It's done like this:-

<xsl:stylesheet version="1.0" 
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
	<xsl:output method="xml" encoding="UTF-8" indent="yes" />
	<xsl:template match="/Parent">
		<xsl:variable name="maxAtrr1">
			<xsl:apply-templates select="." mode="getMaxAttr1" />
		</xsl:variable>
		<xsl:variable name="itemsMax" select="Items[@Atrr1=$maxAtrr1]" />
		<result a1="{$itemsMax/@Atrr1}" a2="{$itemsMax/@Atrr2}" 
a3="{$itemsMax/@Atrr3}" a4="{$itemsMax/@Atrr4}" a5="{$itemsMax/@Atrr5}" 
a6="{$itemsMax/@Atrr6}" />
	</xsl:template>
	
	<xsl:template match="Parent" mode="getMaxAttr1">
		<xsl:for-each select="Items">
			<xsl:sort select="@Atrr1" order="descending" data-type="number" />
			<xsl:if test="position()=1">
				<xsl:value-of select="@Atrr1"/>
			</xsl:if>
		</xsl:for-each>
	</xsl:template>
	
</xsl:stylesheet>