It's not an either or for me.
Flabby or flat are both huge disappointments, but a perfectly round tiny hank hill butt or Nikki Minja Silicon Balloon arse is no better.
Symmetry, a clean distinction between what is bum, back and thigh, while also not looking like a separate body part that's been bolted on, enough fat to have a wobble when you slap it, but enough muscle to hold it's shape and spring back into place after, and soft, smooth skin are all vital ingredients in making a perfect arse.
What exact size that arse is depends on personal taste though, so while shape is a consistent factor, size is a variable, but that doesnt make one or the other less important, it just means what's perfecr for one man won't be the same for the next, but neither's choice is any less 'perfect'