Honestly, I think it's her voice. In no way is it the greatest, but something about the tone just makes every song sparkle. I doubt any of her contemporaries could make a song like "Diamonds" a hit. Like, I can't imagine it sounding good with Christina, Beyonce, Britney, Gaga, etc. singing it. But somehow, Rihanna just makes it amazing

There are just so many otherwise basic and filler songs in her discography that her tone and accent turn into gems. It's actually shocking
