It depends on the size of the person they are attached to. On myself I prefer a B cup but my band size is 42 so a B cup is 850 ml per cup.
See https://www.laurengreenbergmd.com/breast/what-is-the-volume-in-cc-of-a-cup-size/
My sister wears a 38D, but it turns out that that is the same cup volume as my 42B bra. Some slim people just wouldn't look right with a D cup in my opinion; but there aren't many slim people these days so perhaps it doesn't matter. I was sure I read somewhere that the average cup size in the UK is D but the World Population review says it is C and that Norway is C-D. But Norwegian women are generally taller than Britons.
See https://worldpopulationreview.com/country-rankings/breast-size-by-country
So, to answer the question, on people of roughly my build (1.70 m tall, BMI 24.2) I think B is best, with a higher BMI or taller then C.