It would have to be an odd tie between King Slime, the Twins, Destroyer and Plantera.
King Slime because the Slime Mount, Slime Hook and Slime Gun are some adorable items to have.
The Twins are a pain to defeat, but it's all worth it to make a beautiful Optic Staff.
The Destroyer is just plain cool looking and his music is awesome.
Plantera because of the unique way of summoning and fighting her, and she's an important step in advancing through the game (especially as a Summoner).