The Galaxy Hammer is the best hammer in Stardew Valley, and it has between 70 and 90 damage per hit, making it very powerful. You can actually buy the Galaxy Hammer from Marlon in the Adventurer's Guild shop for 75,000g, but you will need to have obtained the Galaxy Sword before he will sell it to you.
So if you don't have the Galaxy Sword, find a Prismatic Shard and bring it to the Desert. Now go north-east and stand in the middle of the three pillars there to get the Galaxy Sword. Make sure that the Prismatic Shard is your active item. This will consume the Prismatic Shard and give you the Galaxy Sword. It's time to head to the Adventuerer's Guild to buy your Galaxy Hammer!