Quite a hot topic, but so OT! I am not sure what to follow exactly...
I cut in because
@radaogapgap posted a list that's unconventional (exclude top ranking idols) so it was interesting. I generally don't participate in any top-6, top-5, top-10 discussion. I wasn't aware of the on-going focus was big tits. My apologies, if I knew the metrics, I wouldn't be as picky on face and age.
It's impossible to come up with 6 names that fit all the requirements: big bust, active, world class beauty, not already top ranking.
Since this thread has gone free-for-all, I'm just randomly doing brain-fart:
To share my tastes: my active tops are: Julia, Shunka Ayami (but wish both would do even more interesting genres). Honoka Orihara (admittedly her face and body not as tight as before, but remember the uncensored 150% score multiplier). Rion (problem is she need to do more interesting genre soon, I'm threatening to un-fan her), Yua Mikami (slightly similar problem, she needs to make more vids and genres), Kotone Suzumiya (formerly Aoi Shirosaki) (she's gone rather inactive, I'm tempted to drop her, but as Aoi Shirosaki she made a few all-time classics both censored and uncensored!) Sora Shiina (she violated many of my demands, small tits, short practical hair, youngish, unglamorous, but there's something about her...)
Retired all-timer: Takako Kitahara (retired for 8 years, but still can't find a replacement for that hole in my heart), Yuma Asami (see below), Maria Ozawa ( not quite as enduring as Kitahara, she was an icon).
Up-and-comer (?) (ok they are not newbie, but I had failed to pay attention to them before)
Natsuko Mishima 三島奈津子
debut at 33 and a half years old, but like her debut cover claims looks like 20.
Ruru Aizawa 逢沢るる

Ok I see you've discussed her and even some fans have dismissed her, but I kind of only (re)-discover her recently, but (checks) wow she's been in the industry 2.5 years and she's only 21!! She looks at least 29 right? Ok she's not top beauty (skin a little dark, thighs a bit scary) but her assets are wonderful. Great big tits, on their own maybe not earth-shattering, but combine with a world class ass, DAMN this is basically a Black Swan event for JAV.
Otherwise: random topics and brain-farts:
Yuma Asami: her face and body are very hot but going by pix only you would not think she's top girl in any period. First you need to know that during her time, big bust were much more rare so you need to rate her titties based on the standard around that time, I'd say her breasts were in the top-5 but if she were active now her pair may not make top-10. But more importantly, much much more importantly, it's her smile and her mannerism and fantastic acting. Near the end of her career she did a sperm donation clinic vid and in the first scene, she received cum in her mouth and she was totally convincing that it was her first time, it was just priceless performance. Her physical looks were maybe 9.5/10 but add her altitude, personality, work ethics (number of vids, length of career, variety of roles). Too bad about her health problem, and I wish she had done at least 5 uncensored vids before retirement...
I don't care one bit that one girl had enhancement and another girl are all natural. One may look better or worse from all kinds of different reasons. I often dislike a pair due to
vaguely they don't look natural, (mostly based on the amount of sagging relative to the volume or weight). The thing is now that big tits are so common, I can afford to be really picky, really skeptical.
I agree with
@Supmop , I don't care about amateurs or even newbies. Firstly JAV don't produce proper amateur sex vids (hobby-like home vids made by the couple or girl herself) what they do is make commercial vids using girls who aren't career AV performers. So you don't get any authenticity since the set, director, etc are all commercial, and you don't get experience and professionalism of a girl who's done vids at least a few times and knows how to perform for the camera. Newbies (with name) are only a tiny bit better, since they are mentally more committed to AV career. But I more prefer girls who are at least 6 months since debut and make a steady output like 1+ full length vid per month. I understand there's a market for "innocent" girls, not I.
Age: I already said I was a bit too picky due to now knowing the big tits premise. But generally for JAV, we can afford to be a bit picky. Asians tend to have good skin that somewhat resist aging and AV is perfectly fine for unrealistic high standard. But "age", IMHO is inaccurate. I want good skin, not young girls. An ideal AV idol maybe 26 years old (debut at 21, 5 years in industry) with skin not noticeably older than 21. A 17 year old girl may have even better skin, but actually I prefer a few more years of experience and maturity and glamourous. A Japanese woman who take care of her body (e.g. just a little but not too much sun, and minimize daily make-up) can look amazing even up to 39 years old. I have only a tiny number of 30+ idols I could even watch, only if they are super hot or more likely they are willing to do what mainstream hotties won't: e.g., uncensored anal sex.
@radaogapgap You are not the only Asian around.