Mythril Sword

A stopgap. You craft this on the way to the mechanical bosses and drop it the moment hallowed bars start arriving, which is often only a couple of hours of play later. It autoswings, and the blade covers enough ground to clear the small hardmode trash that starts swarming you after the wall breaks, which is really what it is for.

Only if melee is actually your class. Everyone comes back from the first hardmode mining trip with a pile of mythril, and those bars go further in a drill, the anvil or a set of armor. If you are still carrying a Night's Edge out of pre-hardmode, the step up here is small enough that the bars are better spent elsewhere.

Its counterpart is the Orichalcum Sword, and whichever one your world gave you, the answer is the same.