The question is about who was the greatest warrior so i have not included Gods as god is greater than humans in all aspects.
Following are the ones who are most close to the human form in Kalyug as kalyug humans are weaker than all other previous Yugs. They are mostly demigods i.e human having god like characteristics.
Indrajeet Meghnath, was the greatest warrior in Ramayana, who could be killed only by Lord Lakshman ji. Indrajeet was a human who got everything perfect in him. He had all the knowledge, strength. He even told.truth about Lord Ram to his father king Ravan and adviced Ravana to Surrender before Ram Ji.
Ganga Putra Bheeshma was the one who had taken the biggest oath ever of not marrying ever in his life. The vow was not only about abstaining from marriage but about being the greatest brahmcharya person. Even the greatest sages of all time in hindu mythology lose control on themselves and break brahmcharya by marrying someone at least once in their life but bheeshma was destined to be the greatest brahmcharya person. It is because of this reason , Lord Vishnu's Avatara Parshuram could never defeat Bheeshma in battle. Bheeshma had converted all the animal energy into spiritual energy by following Brahmcharya.
Suryaputra Karna is the greatest warrior after Bheeshma Pitama. He was the one who had never lost any fight. Even Arjun could never defeat him in battle without Lord Lord Krishna, Hanumanji and Indradev's help. Also when Arjun killed Karna, Karna was unarmed which is against the rules. But the Mahabharat war was not between Arjun and Karn, rather this time it was between Good and Evil. Arjun had followed Dharma all his life as guided by his elder brother Dharm Raj Yudhishthir and Lord Krishn. And Karna had unknowingly commited sins by helping Suyodhana, better known as Duryodhan. Only because Duryodhan had given him love shelter and everything which the Lord Krishna was unable to give Karn.
Karn was not evil but a person who made a big mistake by not realizing the true nature of Duryodhan's evil side. Duryodhan made karna his friend because he knew that Karn was the only one who could defeat Pandavs all alone. Also remeber that Karna was the eldest of all the Pandav and Kaurav brothers thats why he was the real person who should be crowned the King amongst all brothers. Karna had always taken the right decisions.all his life except the one of not realizing the nature of Duryodhan. If had had been crowned the king the whole Mahabharat would have changed, there would be no war and only peace then.
Suyodhan is greatest warrior after Karn. It is only because of is bad nature and greed of throne, the mahabharat took place. Otherwise he had some good traits too. He helped the Sut Putra Karn, (sut means charioteer Or a lower caste according to the old caste sysytem) and gave him shelter and love which Karna wanted to get. He is the one who had been the most loyal person in the mahabharat because he had always helped his friend Karna whenever needed. He was better skilled than 5 Pandavs but Guru Drona loved the Pandavs more than Kauravs which is wrong on part of a Guru. Drona never praised Duryodhan for his skills and this led to rise of jealousy in Duryodhan's mind for Pandavs.
Mahabharat message through the life if Suyodhan is that, even if a person has great skills, it will be of no use if the person is of bad nature . Use the skills for good cause ,not for bad and selfish reasons.
Message through Karna in Mahabharat is that there should.be no discrimination on the basis of caste, gender, colour etc. Karn was discriminated on the basis of caste all his life. Rather he was the most apt person among all the Pandav.and Kaurav brothers.
One and only unbeatable ultimate warrior Karna
According to me Karna is the greatest warrior in all sense. He struggled his life, he is the one whom lord parshuram(Vishnu's, dronas guru whose disciple was arjuna) avatar declared as equivalent of himself in archery. He suffered pain throughout his life but never got diverted from his path. Greatest danveer of all time. He faced all problems with proud.
abhimanyu is the greatest warrior i thought and he is the elder of upapandavas he fought very brave in padhmavuyha mahabaratha
Suryaputra Karn is my idol and I am a biggest fan of his
This poll shows how much people have been misguided by fictional novels and TV Serials,they consider Karna better than Lord Ram,Krishna,Hanuman when he was no match for Arjuna even.