[007julien]

I propose this two lines script with a backreference regular expression

<script type="text/javascript">
//array to test
var arr=[1,41,222,33,40,18,22,41,2,2,41,42,27,41,58];

var lgt=0,nmb='',str=arr.sort().join(',').replace(/(,\d+)(\1+)/g,
	function(a,b){var m;if (lgt<(m=a.length/b.length)) {lgt=m;nmb=b.substr(1)}});
alert('This number '+nmb+' appaers '+lgt+' times');	

</script>