public static int Compare(params IList<int>[] lists) {
var max = lists.Max(x=>x.Count());

for(int i=0;i<lists.Count();i++)
{
if(lists[i].Count == max) return i;
}
return 0;
}

List<int> ls1 = new List<int>() { 1,2 };
List<int> ls2 = new List<int>() { 1,2,3 };
List<int> ls3 = new List<int>() { 1,2,3,4,5 };
List<int> ls4 = new List<int>() { 1,2,3,4 };

List<string> listexp = new List<string>() {"ls1", "ls2", "ls3", "ls4"};
Console.WriteLine(listexp[Compare(ls1, ls2, ls3, ls4)]);