FizzBuzz.cs

              new Dictionary<int, string>
                {
                    { 3, "Fizz"},
                    { 5, "Buzz"},
                    {15, "FizzBuzz"},
                }
              .Where(item => i % item.Key == 0)
              .Select(item => item.Value)
              .LastOrDefault() ?? i.ToString();

FizzBuzz2.cs

                new Dictionary<int, string>
                {
                    { 3, "Fizz"},
                    { 5, "Buzz"},
                    {15, "FizzBuzz"},
                }
                .Where(item => i % item.Key == 0)
                .Select(item => item.Value)
                .LastOr(i.ToString());


static class _
{
    public static T LastOr<T>(this IEnumerable<T> s, T t) where T : class { return s.LastOrDefault() ?? t; }
}