Best food for my little Chihuahua puppy?

Hi! I'm the proud owner of an awesome male Chihuahua. He is now 12 weeks old and I have being feeding him with Purina Dog Chow for small breed puppies. After all I've read on the internet in the past weeks, I believe its time to change his food to something better and healthier. I was tempt to get him Eukanuba, but some of you say its no good. What do you recommend? I have visited some of the websites you posted, there's one problem tho, I live in Santiago de Chile, and we do not have as many brands, or they are not that easy to get, and online buying is not only expensive but also difficult. Would you maybe tell me which components should I look for? I heard I should avoid the by-products, what else? Thank you!!