I think butts are underutilized because most actresses (even american) are really bad at reverse cowgirl. It's just a really difficult position. Momo did it very well in ipvr-86 though so that's why she's #1 otherwise most actresses can't get the entry and exit of reverse/riding, so it becomes a waste of scene time. the kimono/soft tied is a great immersive scene type that I think does well especially if the girl is pretty + busty. (rino yuki, momo, yu shinoda) I don't think there has been any scenes that have the MC smashing the actress and then a bystander is incorporated. Sometimes it's hot when in the first scene she's riding hot and heavy, then in the next you've swapped with a different actress, but the first actress is still seen in the background riding someone else
I don't know if you're a fan, but I'd like to say Sora Amakawa is pretty good at it... and any position where she's on top.