Who was the most powerful warrior in the Mahabharata?

Although he didn't impact the war! Thanks to Krishna's presence of mind it has to be Barbarika, Bhima's grandson.

The legend begins with the Mahābhārata. Barbarika alias Khatushyamji alias Shyam was a grandson of Bhima, Second of the Pandava brothers. He was the son of Ghatotkacha (who in turn was son of Bhima) and Kamkantkata Ma Morwi . Even in his childhood, Barbarika was a very brave warrior. He learnt the art of warfare from his mother. God Shiva, pleased with him, gave him the three infallible arrows (Teen Baan). Hence, Barbarika came to be known by the appellation Teen Baandhaari, the "Bearer of Three Arrows". Later, Agni (the god of Fire) gave him the bow that would make him victorious in the three worlds.

Before the Mahabharata war began, Lord Krishna asked all the Pandavas how many days he would take to finish Mahabharata war alone. Bhishma answered that he would take 20 days to finish the war. Dronacharya replied that it would take him 25 days. When Karnawas asked, he said he would take 24 days. Arjuna told Krishna it would take 28 days for him to complete the battle by himself. In this manner, Lord Krishna asked each warrior and received an answer.
Krishna disguised as a Brahmin stopped Barbarika to examine his strength. When asked how many days he would take to finish the war alone, Barbarika answered that he could finish it in one minute. Krishna baited Barbarika by mocking him for going to the great battle with only three arrows. On this, Barbarika replied that a single arrow was enough to destroy all his opponents in the war, and it would then return to his quiver. He stated that, the first arrow is used to mark all the things that he wants to destroy. On releasing the third arrow, it would destroy all the things that are marked and will then return to his quiver. If he uses the second arrow, then the second arrow will mark all the things that he wants to save. On using the third arrow, it will destroy all the things that are not marked. In other words, with one arrow he can fix all his targets and with the other he can destroy them. Krishna then challenges him to tie all the leaves of the peepal tree under which he was standing, with those arrows. Barbarika accepts the challenge and starts meditating to release his arrow by closing his eyes. Then, Krishna without the knowledge of Barbarika, plucks one of the leaf of the tree and puts it under his foot. When Barbarik releases his first arrow, it marks all the leaves of the tree and finally starts revolving around the leg of Krishna. Then Krishna asks Barbarika, as why was the arrow revolving around his foot? For this, Barbarika replies that there must be a leaf under his foot and the arrow was targeting his foot to mark the leaf that is hidden under him. Barbarika advises Krishna to lift his leg, since, otherwise the arrow will mark the leaf by pricking Krishna's leg. Thus, Krishna lifts his foot and to his surprise, finds that the first arrow also marks the leaf that was hidden under his foot. Of course, the third arrow does collect all the leaves (including the one under Krishna's foot) and ties them together. By this Krishna concludes that the arrows are so infallible, that even if Barbarika is not aware of his targets, the arrows are so powerful that they can still navigate and trace all his intended targets. The moral of this incident is that, in a real battle field, if Krishna wants to isolate someone (for example: the 5 Pandava brothers) and hides them elsewhere in order to avoid them from being Barbarika's victim, then Krishna will not be successful as the arrows can still trace the target and destroy them. So, nobody will be able to escape from these arrows. Thus Krishna gets a deeper insight about Barbarika's phenomenal power.
Krishna then asks the boy whom he would favour in the war. Barbarika reveals that he intends to fight for the side whichever is weak. As Pandavas have only seven Akshouni army, when compared to Kauravas eleven, he considers that Pandavas are weak and hence wants to support them so that Pandavas will become victorious. But Krishna asks him, did he seriously gave a thought about the consequences before giving such a word to his mother (to support the weak side). Barbarika guesses that his support to the weaker side will make them victorious. Then, Krishna reveals the actual consequence of his word to his mother:
Krishna tells that whichever side he supports will only make the other side weak due to his power. Nobody will be able to defeat him. Hence, he is forced to support the other side that has become weaker due to his word to his mother. Thus, in an actual war, he will keep oscillating between the two sides, thereby destroying the entire army of both sides and eventually only he remains. Subsequently, none of the side is victorious as he will be the only lone survivor. Hence, Krishna avoids his participation from the war by seeking his head in Charity.