A brilliant show doesn’t need a brilliant OP, but it can’t hurt to have one, and Shigatsu wa Kimi no Uso has it in “Hikaru Nara” (光るなら; If You Will Shine) by goose house. The song and its accompanying imagery really does shine and shimmer the same way the show and its characters do at numerous points in the episodes. My only complaint is with the somewhat goofy synth trumpet. It’s catchy as all hell; having watched the episode last night, the song popped up in my head the next morning and remains there still.
At the same time, the song balances the joyful with the melancholy, nicely capturing the overall bittersweet tone of the show. Kousei, Kaori, Tsubaki, and Ryouta may experience gloriously perfect moments from time to time, but they’re only moments, within an imperfect reality featuring an unfortunate love rhombus and what could be a ticking clock for Kaori.