I have a hard time to pick any unique item by usefulness, but when it comes to fun I remember the early Eaglehorn was like a drug to play with. Back then it was Twisting Blades meta, so it didnt get much attention by the player base.
But didnt matter to me, it was too hard to put this bow and Penetrating Shot away. Watching the bouncing mechanic do its thing was incredible addicting and it required positional consideration. Affixes were good, and still are.
Unfortunately, the unique aspect only survived 2 seasons. The difference to now is that the chance to bounce was 80%, instead of 25%. I believe this was changed as it was a pain to the servers, but not sure about that.
Good thread btw.