Ippolito's at 13th and Dickenson is excellent. They are "related" to Samuels and son which supplies most of the area's restaurants. They have wonderfully fresh seafood because of the turnover and will get you ANYTHING with about 24 hours notice. They will also cook anything in the store for you in a basic bake/broil/fry preparation.
From West Philly, I'd take the El to Reading Terminal (maybe with a small cooler). I prefer the fish stand that has the 'eat fish live longer' sign, but I think they are all good. Also, they'll debone whole fishes for you if you want. It's worth checking them all out to see which has the best quality and price on what you want any given day. Thank you.
Are you talking about to "eat it in a restaurant" or to actually just buy it in a store?
If the second, I am not sure this is the right thread for your post.
If you meant to dine, I recommend Estia.
For purchase to take with you, believe it or not, Food Source has very fine fish and fresh the same day.