My favorite place for seafood is still Dolphin near Harvard Square (roughly 1100 Mass Ave). They've got huge portions at student-friendly prices. Go for the basic stuff: fresh, well-cooked seafood, broiled baked or fried, with an assortment of plain side dishes done plain. It's definitely the place to take anyone who is serious about just plain good seafood. They recently renovated, raised their prices a little bit and added some fancy stuff to the menu, but the food seems the same underneath.
Boston, MA