Some excellent brands: Innova, Artemis, Wellness, Chicken Soup - which you will find online or at a high quality pet supply store. These brands are not carried at Petco or Petsmart. Petco is now carrying Solid Gold, and that is a pretty high quality food
you can learn about a lot of different brands & compare ingredients at
Dog Food Analysis - Reviews of kibble
A 4 or 5 star food would be best. Some yorkie owners do use EVO which is a 6 star food, but a lot people think the 6 star foods are too high in protein for yorkies.
That website is just someone's ratings, but it is a good place to start
One of the problems I have with Science Diet is that there are a lot of fillers in it
Also, my dogs are not real thick, they are smaller bones, so I don't want to overfeed them.
A lot of the pet food companies will send you samples of their food. You can just email them with your address & they will send it to your door. You can find a sticky at the top of this section with links to a lot of the company's websites, or contact info