Progression in Terraria is defined, in part, by beating bosses. That's why each boss (save the King Slime, Queen Bee, Queen Slime, Duke Fishron and Empress of Light) drops something that's crucial for advancement, or otherwise is necessary for accessing something important. The Eye of Cthulhu drops Demonite/Crimtane (depends on what evil your world has) ore, which is incredibly rare otherwise. The Eater of Worlds drops Shadow Scales, which are necessary for most of the useful Demonite gear. The Brain of Cthulhu drops Crimtane ore and Tissue Samples, required to make high-pre-Hardmode gear. Killing Skeletron is necessary for accessing the Dungeon, where all manner of useful things are found. And killing the Wall of Flesh activates Hardmode, as well as drops the Pwnhammer, which is vital to getting the Hardmode ores.
What's interesting is the end-game loot. There are three Hardmode bosses, and each one drops a specific type of soul. Later on, there's Plantera, Golem, and finally the Lunar Events. You have reached the end of the game's progression when you have defeated these bosses, and thus have access to just about any item you could want.
The ultimate items would therefore be the one that requires, as crafting material, ores or weapons dropped by the very final boss.
And what are they? Is it the Meowmere, the Star Wrath, the SDMG, the Last Prism, Lunar Flare, or the Rainbow Crystal Staff?
They are the Drill Containment Unit and the Zenith.
Both of them represent two aspects about what Terraria is about — mining, and combat. The Drill Containment Unit is the ultimate mining tool, capable of deconstructing anything that can be deconstructed. The Zenith is the hard-earned culmination of various swords obtained and crafted through the game, and the most powerful weapon of all. Thus, the game is truly complete once both items have been obtained, and the player is capable of deconstructing all that can be deconstructed, and defeating all that can be defeated.