Find character with most occurrences in string?

input.GroupBy(x => x).OrderByDescending(x => x.Count()).First().Key

Notes:

• if you need this to work on ancient (2.0) versions of .Net consider LinqBridge. If you can't use C# 3.0 (targeting .Net 2.0) you probably better off with other solutions due to missing lambda support. Another .Net 2.0+ option is covered in xanatos answer.
• for the case of "aaaabbbb" only one of those will be returned (thanks xanatos for comment). If you need all of the elements with maximum count, use Albin's solution instead.
• due to sorting this if O(n log n) solution. If you need better than that - find Max value by linear search instead of sorting first which will give O(n). See LINQ: How to perform .Max() on a property of all objects in a collection and return the object with maximum value

string testString = "abbbbccd";
var charGroups = (from c in testString
                    group c by c into g
                    select new
                    {
                        c = g.Key,
                        count = g.Count(),
                    }).OrderByDescending(c => c.count);
foreach (var group in charGroups)
{
    Console.WriteLine(group.c + ": " + group.count);
}

Inspired from Stephen's answer, almost the same:

public static IEnumerable<T> Mode<T>(this IEnumerable<T> input)
{
    var dict = input.ToLookup(x => x);
    if (dict.Count == 0)
        return Enumerable.Empty<T>();
    var maxCount = dict.Max(x => x.Count());
    return dict.Where(x => x.Count() == maxCount).Select(x => x.Key);
}

var modes = "".Mode().ToArray(); //returns { }
var modes = "abc".Mode().ToArray(); //returns { a, b, c }
var modes = "aabc".Mode().ToArray(); //returns { a }
var modes = "aabbc".Mode().ToArray(); //returns { a, b }

Update: Did a quick benchmarking of this answer vs Jodrell's answer (release build, debugger detached, oh yes)

source = "";

iterations = 1000000

result:

this - 280 ms
Jodrell's - 900 ms

source = "aabc";

iterations = 1000000

result:

this - 1800 ms
Jodrell's - 3200 ms

source = fairly large string - 3500+ char

iterations = 10000

result:

this - 3200 ms
Jodrell's - 3000 ms

This because someone asked for a 2.0 version, so no LINQ.

Dictionary<char, int> dict = new Dictionary<char, int>();

int max = 0;

foreach (char c in "abbbbccccd")
{
    int i;
    dict.TryGetValue(c, out i);
    i++;
    if (i > max)
    {
        max = i;
    }
    dict[c] = i;
}

foreach (KeyValuePair<char, int> chars in dict)
{
    if (chars.Value == max)
    {
        Console.WriteLine("{0}: {1}", chars.Key, chars.Value);
    }
}

Instead this for the LINQ version. It will extract paired "bests" (aaaabbbb == a, b). It WON'T work if str == String.Empty.

var str = "abbbbccccd";

var res = str.GroupBy(p => p).Select(p => new { Count = p.Count(), Char = p.Key }).GroupBy(p => p.Count, p => p.Char).OrderByDescending(p => p.Key).First();

foreach (var r in res) {
    Console.WriteLine("{0}: {1}", res.Key, r);
}