Okay - House of the Sun is already really good, and I'm only on chapter 3...
ETA - just worked out that this is Taiyou no Ie. I thought it was weird that it was not only this good but award-winning, but I couldn't remember ever hearing of it.
I just never heard of it under that localized title.
Due to certain circumstances, Heike Waku transfers to a new high school. There, he meets a very cool classmate named Oura-san. With a famous model for a sister and a face that never smiles, she seems like a distant and untouchable presence… or so it seemed. But by chance, Waku happens to catch a glimpse of Oura-san's dazzling smile. In that moment, his world changes completely! For that smile… I'll do anything!! From Taamo, the creator of House of the Sun, comes a heart-pounding, grin-inducing high school romance!