Group By Multiple Columns / Count And Then Find A Max

I have three tables,let's say

table stores
sid | store_name
1 | one
2 | two
3 | three

table products
pid | sid | p_name
1 | 2 | pone
2 | 2 | ptwo
3 | 3 | pthree

table sales
said | sid | pid
1 | 2 | 1
2 | 3 | 1
3 | 2 | 2
4 | 1 | 3
5 | 2 | 2
6 | 3 | 2
7 | 3 | 2

and i want display the product that sells best in every store. I try to group by multiple columns counting how many times each product was sold in every store, but don't know how to select the one which was best sold (maximal number of times)