A combination of elegant production, angelic vocals, and most importantly, an infectious, beautiful melody. It's melodically one of the best songs of all time. The way she sings "It's the way I'm feeling, I just can't denyyyy" is so unexplainably amazing and infectious.
Mostly because of Calvin's production. While i like Rih, this song exactly is not as good because of Rihanna, it's not even that good overall.
Now if we talk about why Diamonds is so good - Rihanna's vocals clearly (and Sia of course). But WFL - Rih doesn't really impress etc. But her voice is still nice in it.
It's her voice. Calvin did an amazing job with the production/lyrics. But Rihanna brought it to life. It's simple, but effective. Her emotional tone carries the song. In it's simplicity there is a powerful message about love. Finding love in a hopeless place and not being able to recover from it.
