I have no question in my mind about that answer. Social-emotional development, including recognition of emotions, coping skills, expression of emotions, empathy, compassion, and 'friend'-skills are the most important thing a child needs to learn before kindergarten.
Humans, in general, need these skills more than any other in order to survive. Solid social and emotion skills will help anyone be more successful in their careers and relationships. Think about that seemingly brilliant co-worker or family member of yours that has an IQ through the roof BUT you can't stand having a conversation with them. Or that family member who can't help you with the crossword but always knows the right thing to say when you are upset.
I have little doubt that my child will learn to read, write, and do math in elementary school. I am confident the fine kindergarten and first-grade teachers of the world will teach children letter recognition, phonetics, number placement, basic addition, the scientific process, and much more. If your child is already interested and knows some of these skills, that is great, but they are not necessary to enter kindergarten successfully.
What is necessary? Being able to navigate the social environment and make sense of it. Asking peers and teachers for what they need. Telling a parent or teacher when they are having difficulty with an academic concept or social situation. Being able to dress, feed, and toilet independently. Being able to learn the academics being presented even if emotionally they are upset. Seeking and giving comfort to those around you. Being able to calm down and focus their energy and attention to a teacher-directed task. Being able to make and maintain friends in their peer group. Learning to balance 'work' and 'fun'.
With my own children, academics has never really been a problem. Our issues always reside in the social-emotional realm. When my daughter first started elementary school, she never ate lunch because she would talk the whole time and then her friends would finish eating and leave for recess. Since there was not an adult sitting with her, urging her to eat, she had to learn to balance her conversation and eating as well as being emotionally ok with remaining at the table to finish while her friends left.
Recently, the now-seventh grader, saw a huge drop in her grades while she and her two best friends had a major falling out second quarter. She had a big challenging figuring out how to focus on her academics while she had to sit next to someone she was fighting with outside of class. This is a good example of how a child without the appropriate coping skills can hurt his academic development. If a child cannot cope with an intense emotion or situation, they cannot learn the information being presented to them. This includes recognizing the emotion or problem as well as finding or asking for help to resolve it.
We, as parents and educators, need to fill the social-emotional toolbox for our children so they can fill up the academic toolbox on their own.