whats the best back exercise for scoliosis

This is a very complex question. And of course, it partly depends on whether it is a functional or structural scoliosis, and whether they have fusion, and how exacerbated the curvature is. There is not one single exercise for someone with scoliosis… remember that that person is not equivalent to their scoliosis, so they need a variety of exercises, as does any other human. They do need to pay attention to their spine, to avoid compression, and to stretch and strengthen to try to create more balance. I particularly like to do one sided work, as then I can modify how much to do on each side.

I would like to recommend you read some work by Elise Browning Miller. She is a wonderful writer who has written on yoga for scoliosis for decades.