Everyone's " the best speakers for vocals" will end up being "the best speakers", which tends to get messy.
Personally, I think one of the great things about vocals are the nuances of a singer controlling his or her voice - how the singer manages diction, decay, or pitch change (either a fade or a break), and other things at the edge of music. These factors are part and parcel of a singer's artistry and to hear all the nuances one may need sensitive speakers. Large horn speakers do it for me.