This is just the mode. Here's a quick, non-optimized solution. It should be O(n).

function mode(array)

{

    if(array.length == 0)

        return null;

    var modeMap = {};

    var maxEl = array[0], maxCount = 1;

    for(var i = 0; i < array.length; i++)

    {

        var el = array[i];

        if(modeMap[el] == null)

            modeMap[el] = 1;

        else

            modeMap[el]++;  

        if(modeMap[el] > maxCount)

        {

            maxEl = el;

            maxCount = modeMap[el];

        }

    }

    return maxEl;

}