You'll get a thousand different opinions on this, LOL.
The best food for your Yorkie is the one they do best on, ultimately.
I've primarily fed Acana to my boy for 7+ years (we've rotated a few times with other brands as well but Acana has been our "staple" food). Always had great luck with it even though I've been annoyed with ingredient changes they've done sometimes, ha! But I usually feed the Acana Singles line and we rotate between some of those flavors.