'Wild Cherry Round Up' is supposedly good for getting rounder form. I wouldn't grow it myself, let alone use it for hybridizing; both of its parents are rust buckets, and any daylily I ever grew that had it in its background, ended up being rusty. I now do ancestry checks on (most) daylilies that I am considering for purchase, and if that plant is in the background, I won't buy the daylily.
If you don't care about rust, though...
