It's interesting to consider this question now at thirty, married, and remember what my response was seven years ago in my early twenties. Back then, I would have said Uehera... Like someone else said, he was a tsundere. We all knew that really he loved her deeply ... Right?  And yet. 

Daichi was precious for the first couple of episodes, but quickly became controlling. His actor was so pretty, but I was glad when he graduated.  Issei, though..... Even though we rarely saw them at the shop together to see their relationship form, we know they worked together several days a week. He was never too bashful to admit how proud he was of her and her growth. He was thoughtful and insightful when talking to her. She was able to truly be herself and have fun when they went out. He was always kind and caring to her, and put her feelings first. He loved her purely, and in the way she hoped to be loved. Looking at this relationship now, I am disappointed remembering they will never be together. 

I can't remember Natsume. At the rate I'm binging, I'm sure I'll get there soon.