var str = "adadfdfseffserfefsefseeffffftsdg";
var maxLength = 0;
var result = "";
while(str!=''){
    oldStr = str;
    getStr = str.charAt(0);
    str = str.replace(new RegExp(getStr,"g"),"");
    if( oldStr.length-str.length > maxLength){
        maxLength = oldStr.length-str.length;
        result = getStr + "=" + maxLength;
    }
}
alert(result);