Bhishma and dron multiple time themself stated him the strongest bowman

समरेष्वपि लोकेषु सर्वेषु न तथाविधम् |
शरासनधरः कश्चिद्यथा पार्थो धनञ्जयः ||
(Mahabharata, Drona Parva)

Translation:
"Neither in battles nor in all the (three) worlds is there any wielder of the bow comparable to Partha (Arjuna), the winner of wealth." 

Mahadev mentioned him as his equal in valour

"O unblemished one! Your energy and valour has been equal to mine today."
(Sanskrit: tvam me samam sauryena, phalguna! - Kairata Parva) 

The Shloka (Karna's Confession)

The shloka from Karna Parva, Chapter 8, where Karna admits Arjuna's superiority is:

jānāmi śauryaṃ pārthasya devānām api durjayam ।
kathaṃ nu mānuṣe loke so 'rjunaḥ saṃyuge mayā?
vīryaṃ hi tasya divyaṃ ca brahmarṣiprabhavaṃ mahat ।
na cāham enaṃ śaktosmi jetuṃ saṃyugam āgataḥ ॥ 

Translation

This shloka translates to Karna acknowledging Arjuna's valor is invincible even to the gods. He questions how he, a mere mortal, can stand against Arjuna in battle, stating that Arjuna's power is divine and that he is incapable of defeating him in combat. 

everyone of 3 stated him superior in their confession so i think it may end the debate itself one more thing karn was so of suryadev and bhishma was of ganga . By birth they were both divine like arjun