C++ :

#include <bits/stdc++.h>
using namespace std;

int main(){
	string s;
	int i,x;
	getline(cin,s);
	int a[200] = {0};
	for(i = 0;i < s.size();i++){
		x = s[i];
		a[x]++;
	}
	
	char c;
	int max = 0;
	for(i = 'z';i >= 'a';i--){
		if(a[i] > max){
			max = a[i];
			c = i;
		}
	}
	cout<<c<<endl;
}

Python :

str1=input().strip()
alist = list(set(str1))
alist.sort(reverse=True)


dic1=dict()
for t in str1:
    if t not in dic1:
        dic1[t]=1
    else:
        dic1[t]=dic1[t]+1
'''
for key in dic1.keys():
    print(key,dic1[key])
'''
bb=sorted(dic1.items(), key=lambda kv: (-kv[1], -ord(kv[0])))
'''
for item in bb:
    print(item[0],item[1])
'''
print(bb[0][0])