As many have said, it depends. I travel across the country on occasion and try to spend some time in pet stores when my wife is (ugh) shopping. I have seen good and bad in all varieties of stores including chain pet stores, big box stores and the mom and pop outfits.
I am fortunate in my home area. The PetSmart has 2 really good people working in the aquatics section. The PetCo has a young man there that is a real fish enthusiast. One of the local stores has been there for 30 years, so they are very good. And, finally, the Walmart in the next town over has a good man there that happens to be my son. He tells me about how people mess up his tanks and it really upsets him. He isnt afraid to tell people if there is a problem with some of the fish or if they are making poor choices.
Best reasons to go to them (at least my local ones)?
PetSmart - lots of good fish on sale almost every week.
PetCo - Some selection that PetSmart doesnt have
Wet Pets - More 'different' varieties of fish like my strawberry and jade tiger barbs.
WalMart - Best prices for fish food and occasionally some decorations. Their selection isnt much different than PetSmart but the fish cost more there.