Arroz con pollo is the "most common rice dish" in Spanish speaking countries.
Explanation:
Arroz con pollo is one of the most commonly eaten rice dishes in Spanish countries. It is a recipe of major ingredients including chicken and various condiments.
It is eaten in most "Spanish speaking countries", viz. Spain, Mexico, Puerto Rico, Costa Rica, Cuba, etc.
It is either cooked and eaten in its original traditional form or by making a few modifications to it depending on the regional preferences.