Music categories and labels are somewhat subjective. I consider ABBA to have been not a Rock Band---the OP’s category, but a Pop Group, and a great one. Benny & Bjorn wrote GREAT Pop songs (chord sequences, melodies, harmonies, arrangements, etc.), and the members had good voices, Frida Lyngstad in particular. She has made great music since leaving ABBA as well; watch her You Tube video of the great Jackie DeShannon song "When You Walk In The Room". Glorious! ABBA songs have been celebrated in Broadway shows for a good reason---they're real good ones!
