MostFrequent · Software Engineering Wiki

Returns the most frequent element of a collection.

Use IEnumerable.GroupBy() to group values by value.
Use IEnumerable.OrderByDescending() in combination with IEnumerable.Count() to order the results in descending order based on frequency.
Use IEnumerable.First() to get the first element and return its Key property, which corresponds to the element’s value.

using System.Collections.Generic;
using System.Linq;

public static partial class _30s 
{
  public static T MostFrequent<T>(IEnumerable<T> values)
  {
    return values
      .GroupBy(v => v)
      .OrderByDescending(v => v.Count())
      .First()
      .Key;
  }
}

int[] nums = { 1, 2, 3, 3, 2, 3 };
List<string> str = new List<string> { "a", "b", "b", "c" };

_30s.MostFrequent(nums); // 3
_30s.MostFrequent(str); // "b"