I saw many of th answers but not all ot them are accurate, so I decided to answer the most accurate answer.
10. JAYADRATHA
*SURPRISED ?*
He was the one responsible for ambhimanyu's death, and lets not forget he had given boon by lord Shiva that he can't be killed by anyone except combination of arjuna and krishna.
It was impossible to kill him because of the boon also given by his father Vriddhakshatra.
9. Abhimanyu
He was Youngest warrior in mahabharata
He was only 33 years old when he died compared to other warriors who were 100+
Full armed Abhimanyu was impossible to be killed
He was son of chandra (moon)
So karna, jayadratha, aswathhama,kripacharya broke his armor and was killed in chakravyu
8. Ghatotkacha
He was Half human and half rakhsasha. Ghatotkacha was invincible because of his huge body and power
So instead of using shakti astra on Arjuna given by lord Indra to karna, karna used this astra to kill him. When he fell down after being killed, he took entire army behind him by falling on them.
7. Duryodhana
His whole body was of stone except his waist area everyone knows why
He was one of the greatest mace warrior on earth
He was strongest of all kauravas and he was still alive for 3 days after all bones being shattered by bheema.
6. Aswathhama
It is believed that he is avtar of lord shiva
He is immortal because of curse given by shri krishna that he will live for 5000 years on earth
He also killed all uppa pandavas (not in war)
He is also responsible for abhimanyu’s death
Being son of drona and avtar of shiva and immortal he was impossible to defeat.
5. Bheema
There can be tie between aswathama and bheem.
Bheem was spiritual son of vayu god and half brother of lord hanuman
He had most kills in the war and he also killed all 100 kauravas .
He was incredibly strong, he also had power of 10,000 elephants given by serpent king vasuki
4. Arjuna
Here comes one of the greatest archer to be ever born on earth.
He was spritual son of lord indra and had bow named (gandiva) given by lord indra.
His charioteer was the god sri krishna himself 8th avatar of god of universe sri vishnu
But remember he was victorious only because of sri krishna or else pandavas would be defeated
3. Dronacharya
As i said pandavas only won because of sri krishna
Power of drona is much more than arjuna
He was 3rd incarnation of lord brahma and teacher of pandava and kaurvas.
Full armoured drona was impossible to be killed
he had sword name (asi) given by lord brahma
He also had knowledge of all three astras- brahmastra, pashupatastra , narayanastra
He was killed by cheating or kauravas would have surely won.
2. Karna
Here comes the greatest man who ever lived on earth
Karna was spiritual son of surya (sun god)
He was given armour (kavach n kundal) given by sun god which was attached to his skin at the time of birth
his armour was taken by lord indra but then also he fought vigorously. Imagine with armour he would be impossible to be killed
He was disciple of lord parshurama and had curse given by him that he will forget all his knowledge at the time of use.
He was insulted by everyone because of being sutputra (later found that he was not sutputra but eldest of all pandavas) he lived his life only because of duryodhana his best freind.
1. Bheeshma
Bheeshma was immortal because of boon given by his father shantanu of ‘ichha mrytu” (he can only die if he is willing to)
Bheeshma was son of goddess ganga.
The power of parshurama (7th incarnation of vishnu) and bishma was same. He was also disciple of parshurama
It was so hard to defeat him that lord krishna who became charioteer of arjuna, have to himself take out the wheel of chariot to kill him (because he vowed before war that he will not use any weapon to kill anybody nor he will kill anyone) so he broke that vow.
Arjuna and shikandi penetrated 130 arrows in his body still he was alive because of boon. he knew that he was at side of adharma (sin) so refused to fight and died after 58 days of war when parikshit (son of abhimanyu) was declared as new king.