Do you mean back packs where the racquets are stored flat against your back when carried, as opposed to the ones where racquets are stored perpendicular to one’s back??
I have prefer the former and am currently using a model by Victor. Before it, I used one by yonex and kawasaki (I think??), both of which have discontinued those models.
I find this sort of backpack good, they are less bulky; it also means that they generally have less storage space though, so I can get maybe 4 racquets in comfortably with gear and shoes