The question is “Who among the four, Vidur, Arjuna, Draupadi and Abhimanyu most deserved to have Krishna’s affection? Why?”

All four of them loved Krishna and deserved to have Krishna’s affection. And they did. Krishna was very affectionate towards all of them.

If the OP actually wanted to find out whom did Krishna love the most, I would have to go with Arjuna. They were eternal companions, together since yugas. No one could take his place in Krishna’s heart. They were basically one. After Arjuna’s death, he went to Narayana’s abode. Forever together.

If you take the spiritual aspect out, then, even on just human level, Krishna loved Arjuna very much. Arjuna was the person he could hang out with. They were the same age, shared similar hobbies and interests, had similar aims and goals. Its automatic to bond with someone with whom you have more in common. In those conservative Vedic times, elders were revered, so Krishna could not just ‘chill’ with Vidura or even Yudhistira for that matter.

Similarly the youngsters like Sahadeva or Abhimanyu couldn’t be that way with Krishna. It wasn’t appropriate. Krishna and Arjuna were also liberal with women. When Sanjaya went to their quarters, Arjuna had his feet on Draupadi and Satyabhama’s laps. Basically, they shed societal regulations and were very free with each other. So Krishna loved Arjuna the most. He loved the others as well, but just not the same way as Arjuna.