I dont agree with you that Arjun was better human and best warrior.
Comparing the fate both had, i feel karna was much powerful and superior than Arjun. Without Krishna, Arjun was nothing. He was like more "projectedSarvshreshta" rather than in "actual".
Karna, even though abondoned by her mother Kunti, and his upbringing in lower cast familiy, he came up to that level that even Krishna was scared of him ( that he can kill the Arjun). To weaken Karna, gods played politics and ensured that he got three major curses (i.e. one from parshuram even though he was innocent, By a brahmin and the earth godess). Even just before the battle, krishna ensured that karna "Kawach" or "Kundal" to which even brahmastra or even krishna sudarshan couldnt pass, should be taken away. And just before the battle, to weaken him psycologically, krishan disclosed to him that he is the brother of Pandavas, and offered him the position of King after battle if he joined him. But karna, a true human being, a true friend didnt leave those who helped them in his bad time. Even kunti did the same and got 'Vachan" from him that she will always remain the mother of Five. In the battle ground, he kept his promise and didnt kill all pandavas, including Arjun (coz sunset was happened just in time, may be other trick from krishna) and spared their lifes even he had a chance to do so. And look how he was killed, If Arjuna was the best and was with the so called GOD krishna, why he killed Karna in the position in which he was armless.
He was the biggest donor till today and donated his "Kawach" or "Kundal" even though he knew that it is Indra who is asking for it.
there are many instances like these but these are the major one. In complete mahabharatha, i believe the Karna character was the best.
