May I add something more?
Being italian, of course I can't say anything about the best shrimp sellers...but, if this rule is the same the world over, shrimps and prawns are really good only when they're fresh and whole (when I can, I buy them still alive!). Be cautious when they are headless, shelled or frozen...they can be satisfactory if the seller is a good one, but have nothing to do with the fresh things! As for the precooked one, I agree with Jim...avoid them if you can.
When I can get the best quality, I like more shrimps and prawns simply steamed for 4-5 mins and then shelled and served with few drops of lemon and good olive oil. Generally speaking, I think that seafood is better when cooked whole than shelled, and that steaming is better than boiling...and this isn't an original thought!
My favourite seafood pasta recipe is this one-simple but tasty:
-Sautè in a large pan with good olive oil a couple of fresh diced tomatoes, a garlic clove and a chili pepper (optional) with the heads of 1 lb shrimps or prawns;
-Remove the heads, add the seafood (shelled or not, according to your taste and the patience of your guests), cook them for 4-5 mins, season with salt and a handful chopped fresh parsley and basil;
-Boil in salted water 14-15 oz spaghetti or linguine, drain them slightly undercooked and sautè them into the seafood pan until ready.
This serves 4.