Hot take, but the Phoenix Blaster. You may be very confused and question yourself why a post-Skeletron gun with solid dps that is easy to craft is mentioned, well it's because there are simply better options available. After Skeletron, the only boss you have left in pre-Hardmode is the Wall of Flesh (Unless you fight Queen Bee after Skeletron) and it's safe to say that even with meteor shots (which are piercing) it's not very good against it. The Hellwing Bow, Molten Fury and the Bee's Knees are all simply much better weapons. And for single-targets, both the Quad-Barrel Shotgun and Boomstick do better as whilst they're slower and have a spread, they do shoot more bullets simultaneously making them deal much more damage per shot.
You could argue that it has higher dps than both of the guns which were listed and that's fair imo, but it's still worse than the Molten Fury which is obtainable earlier and is also just easier to get. The Molten Fury also has the advantage of being a bow, meaning it can fully utilize Jester Arrows which are broken in pre-Hardmode.
Really the only thing it has going for it in my opinion is that it's good against regular enemies and is okay with dealing against early-Hardmode enemies from my experience.
Now is it the most underpowered weapon in the game? No, that's for sure, but it certainly is a stinker compared to the ranged weapons of its tier imo.