I am actually impressed with most of my petco purchases recently. I've bought an emperor angel, Queen angel, powder blue tang, red coris wrasse, magnificent foxface, blonde naso tang, solar fairy wrasse, exquisite fairy wrasse, and a couple others.
The only one I was very unhappy with was the powder blue tang. He was skinny as heck, so stressed he was literally translucent and had a sunken stomach. He didn't make it 24 hours. The blonde naso lived one week and then died randomly.
So I guess with my experience I would say avoid tangs but particularly powder blues. Everything else was pretty great.