The Swedish quartet ABBA were pop perfectionists, wielding sterling harmonies in a way that added just the right amount of pathos to their grandiose tunes. They could bring almost any emotion onto '70s dance floors—celebrating loneliness on “Dancing Queen,” singing jubilantly of breakups on “Mamma Mia,” and turning Napoleon's surrender into a storming anthem on the Eurovision smash “Waterloo.” ABBA were masters of blending: mixing disco and rock, balancing sadness with joy. The glittery songs that resulted are still pop's gold standard decades later.