Drona - I know only one which is Brahmadanda which contained the powers of the Saptarishis ,Drona used it on the 15th day and no-one was able to stop it not even Arjuna because Drona never imparted the knowledge of this weapon to Arjuna or Ashwasthamma. He had one more named NarayanAstra it was given to Drona by Lord Narayan himself .
Arjuna - Pashupatastra - I think it was exclusive to Arjuna ( correct me if am wrong ) and it was the most powerful weapon anyone could possess it was the power of Rudra the destroyer of the universe himself.
Karna - He had two as far as i know which were exclusive and nobody else possessed , first was Bhargav Astra which contained the warfare skill of Parshurama Himself , and Vasav shakti ( also called Vaijaynthi or Bashab shakti ) it was given by Indra to Karna because Karna gave his Kavach and Kundal this Astra was never known to mankind , This astra was so powerful that if the Person who possessed it Calls out the name of the person he wants to kill No-one in three world could stop this astra from killing him.
Bhishma - He has Prashwastra This weapon can put his Foe to sleep , This weapon was the reason Gods Stopped Bhishma from using against his Guru because his Guru would have been humiliated.
Correct me if am wrong.