Eccentrically unique music style, especially with earlier titles such as Peek-A-Boo, Ice Cream Cake, and famously Dumb Dumb. I certainly prefer the newer 'mature' styles such as Psycho and Bad Boy, but Red Velvet have an amazing and diverse discography that can be targeted at many demographics.
They also have beautiful members who all have distinct voices (which deliver stunning vocals) so they're a complete package of a group.
They're easily in one of my top 3 groups, but the order of those top 3 really changes often!