Yes, pretty much every boss has a unique item, here is the list:
- PUMIN SEAL
- Boss: Tarbaz

- Unique loot: Shroud of Despair (Arm: 7, protection death +3%).

- TAFARIEL SEAL
- Boss: Shulgrax

- Unique loot: Treader of Torment (Arm: 4).

- VERMINOR SEAL
- BAZIR SEAL
- INFERNATIL SEAL
- Boss: Mazoran

Unique loot: Impaler of the Igniter (Atk: 25 + 26 fire, Def: 31 +3). 
Unique loot: Tempest Shield (Def:36). 
- APOCALYPSE SEAL
- Boss: Razzagorn

- Unique loot: Visage of the End Days (Arm: 10).

- ASHFALOR SEAL
In my opinion, the best boss to farm every 2 days is Mazoran, because it can be done EASILY with just 2 players, you don't rely on a lot of people, so you can just run feru and kill it, will take 15 min. And it has 2 nice items as loot, instead of just 1.