Infinity +1 Sword

The Zenith: every era of Terraria's swordsmithing folded into one screen-filling rainbow blender. The recipe (at a mythril anvil): Terra Blade (Sword of the Hero), Meowmere and Star Wrath (Moon Lord drops), Influx Waver (Martian Saucer), The Horseman's Blade (Pumpking), Seedler (Plantera), Starfury (Floating Island chests), Enchanted Sword (surface shrine shards) and the Copper Shortsword you were born with (the Merchant sells one if you binned it).

It's a scavenger hunt across the whole game - the Moon Lord drops and the Enchanted Sword hunt are the usual stragglers. Forge it and nothing that walks, flies or bosses will trouble you again.