What food should I feed my shih tzu puppy?

Hey there, I have a shih tzu pup that'll be 8 months in a matter of days. However, my grandma's caretaker keeps feeding her Beneful. I've actually seen how people on yahoo answers have rated this product and it doesn't look great :( currently, she eats iams puppy food and lots of ppl have said iams isn't great too. I actually went to petsmart and they told me royal canin is great for my dog when she grows up. Is that true? and also, what do i feed my dog? by the way... my dog often gets poop stuck at her back. is because of beneful???????