Who is more loyal karna or arjuna?

There were lots of greatest warriors in the Mahabharata, among them Karna and Arjuna were also great as a warrior. As they were Superior in the battlefield, they were more loyal, dedicated, kind hearted too. 

Karna, the son of suryanarayan(sun) was most dedicated in his ambition. He faced lots of challenges during his life for getting a knowledges about sword because he was belonging from lower-class family (foster family). He was obligated to change his appearance as a brahmin and went to Parshuram for getting education. After learning there, he returned to hastinapur and made a friendship with duryodhana(Son of Dhritarashtra) because the Dhritarashtra's son gave karna a small part of country and announced a king of that. Karna was powerful warrior so duryodhan gave his hand for friendship and Karna got something that is beyond his imagination, so he made relation with duryodhan as a friend. Karna never broke his friendship with duryodhan till the death even he knew he was doing sin  along with duryodhan.

On the other hand, We all know that Arjun and Shri krishna was very good friends as well as relatives too(cousins). One day God Krishna was sleeping , both aruna and duryodhan went to him to beg the arm forces and power. Arjun was sitting at the feet and duryodhan at the side of head. After waking up, Shri krishna saw arjun first, so he gave arjuna first chance to ask what he wanted from him. Arjuna said "I need Shri krishna" even he knew Shri krishna won't take  part in war and he will just be a charioteer. Duryodhan choosed Senas ( army ) from Sri  Krishna. From these story, duryodhan was opportunist and arjuna was loyal to his best friend.

To sum up, both Karna and Arjuna was loyal in their own perspective. Arjun didn't think to get benefit from Shri krishna but incase of Karna it was benefitted friendship with Duryodhan.