Doesn't really matter, any size would be just fine, even A cup or loli size...
Well, you will learn not to be fussy over it if you're in my country/place, about 80% of the women that you'll encounter are super fit/thin = small tits, Great bodies though, breasts excluded..
Myself, I don't really have an actual preferred size, though if I really had to pick one, I'd say C-cup. I'm ok with most things, as long as the whole package works in general. The one thing that I actually dislike is the stretched out, sick looking balloon breasts that are released into the wilds due to excess in implant use (think Courtney Stodden and her like).