var res = maxRatingBigrams.Min(s => s.Key);

var res = maxRatingBigrams.Min((x, y) => string.CompareOrdinal(x.Key, y.Key));

SortedDictionary<string, int> dict = new SortedDictionary<string, int>();

        dict.Add("aaa", 5);
        dict.Add("bbb", 7);
        dict.Add("ccc", 8);
        dict.Add("ddd", 8);
        dict.Add("eee", 4);
        dict.Add("fff", 4);

        int max = dict.Values.Max();
        string keyMax = "";
        foreach (string key in dict.Keys)
        {
            if (dict[key] == max)
                keyMax = key;
        }

        Console.WriteLine(keyMax.ToString() + dict[keyMax]);