MaxBy

public static TSource MaxBy<TSource, TProperty>(this IEnumerable<TSource> source,
    Func<TSource, TProperty> selector)
{
    // check args

    using (var iterator = source.GetEnumerator())
    {
        if (!iterator.MoveNext())
            throw new InvalidOperationException();

        var max = iterator.Current;
        var maxValue = selector(max);
        var comparer = Comparer<TProperty>.Default;

        while (iterator.MoveNext())
        {
            var current = iterator.Current;
            var currentValue = selector(current);

            if (comparer.Compare(currentValue, maxValue) > 0)
            {
                max = current;
                maxValue = currentValue;
            }
        }

        return max;
    }
}