we have 3 walmarts and a meijers within about a 45-50 mile radius and none of them are very good looking in the aquarium sections. usually the walmart to the east has alot of fish with ich and/or dead fish, fish eating other fish, fish missing pieces, etc. the meijers right across the street from them isnt quite as bad, and ive actually been eyeballing a long finned male swordtail thats been living there for about 8 months... i dont think anything else has lived there that long, but hes always there.
the walmart right by my house has a fairly knowledgable department manager, but shes not there too much and if shes gone for a few days i usually notice all the fish have ich. nobody else there knows anything at all about fish. one time she wasnt there on delivery day and i saw them put a batch of corys in with a handfull of cichlids.... what a mess of shredded fish that was. they thought they were lace cats.
the walmart to the west usually has fairly good looking fish, but they also sell violet goby's as a fresh water fish, wich makes me sad.
then theres 2 LFS that i visit. the one by my house has been in business since before i was born (so better than 25 years) and are usually pretty on point with their advice and have nice looking tanks. the fish are healthy about 90% of the time and i try my best to purchase there when i need to purchase, although they make it hard at times because they wont order from alot of the distro's that carry what i want for special orders and wait for months on end to get it from their regular distro.
the other LFS (45min north of me) has an awsome selection and always gets exactly what i order, when i order it, but they arent afraid to use wal-mart's distributer either, so they end up with alot of sick fish. their pictus cats and chanel cats are usually very very sickly and covered in ich. their prices are OUTRAGEOUS as well. their blue/gold/pearl/etc gouramis are usually around $12
now the only petsmart is about 2-3 hours north... ive been there two or three times in the last 5 years or so. their variety blows, their sales persons blow, their knowledge is minimal and their fish are hit and miss. i have a clown loach and a striped peacock eel i purchased there and theyre doing pretty well and ive had them for about a year... but some of the fish didnt look too good. the balas had ich very bad and there were a handfull of other fish that were dead and nobody really cared i guess. the only thing i liked about purchasing from petsmart was the fact that the snotty i-hate-my-job acting woman that bagged the fish for us asked us in a monotone voice if we were aware that the fish would reach so many inches. couldnt say much for her personality, but at least she pushed out what the store more than likely forced her to say, haha.
as for petco, i have never seen one or been in one, so the only knowledge i have of their stores is what i read here.