Okay, let's do this. Keep in mind that I have not yet seen some Hardmode stuff, so a lot of my knowledge will be pre-Plantera Hardmode (although I have read so much on the wiki and seen so many playthroughs that I know a lot).
Favorite Boss: The Twins (it is just a really interesting fight, and I love their design)
Favorite Event: Lunar Events (they are incredibly amazing, and I am looking forward to doing them in the future)
Favorite Enemy: Mimic (who doesn't love Mimics?)
Favorite NPC: Goblin Tinkerer (that guy has way cool stuff)
Favorite Weapon: S.D.M.G. (I am a Ranged class, so this gun looks legendary)
Favorite Biome: The Crimson (the atmosphere of that biome is so fitting)