Hello!  I, too, came here looking for the best food for my yorkie.  It is SO much more complicated than you would think!  
 
Nick came from the breeder on Purina Pro Plan - from there I switched him (like an idiot) to Science Diet, Beneful, Nutro Ultra, Natural Balance, Innova and now he eats Evanger's Pheasant and Brown Rice.   
I had NO clue about dog food and honestly, bought what ever bag I liked at the time.  After joining YorkieTalk and getting SO much needed advise I learned what to avoid and what to look for in a good kibble.   
What it boils down to basically is you will NOT find a quality, healthy food at a supermarket or grocery store.  They all are low quality and contain fillers, by-products and other gross stuff.   
You want to find a kibble that does not contain corn (in any form), by-products or fillers.  A good site I used when I was searching was this one: 
Dog Food Analysis.  You can see reviews on just about ALL dog foods.   
I went with a 5 star food as all the 6 star are super high in protein - a little too high for tiny yorkies in my opinion.   
Plus, they make kibble called All Life Stages.  That means you can feed to a puppy or adult - they are formulated for every stage of life.  This is what I chose because I have an adult and puppy that eat from the same bowl.  This also prevents having to switch to an "adult" formula later on.   
What I did was I contacted several high quality brands and asked for samples.  There is a list with contact information at the top of the Health & Diet section.  Some will send through mail, others direct you to a local retailer that stocks samples.  I found one Nick and Daisy both liked and went with that.  The first one, Innova, didn't work out for us - made Daisy's poop horrendous.  So, now we are on Evanger's Pheasant and Brown Rice and both pups like it and are doing well.   
Sorry for such a long response - but it really IS more complicated than you would think!!  Anyway, good luck in your search!