Located on magnificent Rittenhouse Square, Devon Seafood Grill is a top-notch place for seafood. I love seared scallops. Devon's are the best in town. They have a great wine list, and the space has a warm, inviting feel. It's not too loud either.
Estia is a Greek restaurant, but in addition to the stunning lamb and other dishes that they do here, they do fish better than anyone else in the city of Philadelphia. You can get a whole fish (yes, with the eyes) grilled to perfection, but they will bring it to you to show you how it has been flawlessly cooked, then take it away and take the fish off the bones for you. Easy! Fish is served with a magical emulsion of olive oil and lemon. I gave up trying to duplicate it at home a long time ago. It's so good that you will want to get some for an IV drip.
Before you have your main fish course, of course, you should have an appetizer. The grilled octopus is the best that I have had anywhere in the world. They have a front-loading clothes washing machine that has never held clothes or detergent. Rather, it is filled with octopus, cold water, and sea salt to gently tumble and therefore tenderize the octopus before it is grilled to perfection. Cooking octopus is like cooking squid: if you screw it up, it is slightly more tender than rubber tire inner tubes. There are no such worries at Estia: after charcoal grilling it to perfection, they dress it lightly with an vinaigrette dressing, toss it with red and sweet onions, dill, roasted peppers, capers, and parsley and serve it immediately. It has the consistency of delicate, succulent chicken. Literally the best octopus I have ever eaten anywhere in the world.
A wide variety of oysters that are always ridiculously fresh, served in a hyper-clean, unassuming, friendly restaurant. Having been introduced to oysters by a French former colleague (I miss you, Yann...), I only eat my oysters with mignonette sauce and lemon. They serve cocktail sauce, of course, but I consider it a sacrilege.
Anyhow, the cooked seafood at the Oyster House is always amazing. It is a truly great place.