>What is the best overall studio?
>inb4 hagchads say Madonna just because they have the best hags
Madonna just because they have the best hags
Jokes aside, it's not really a helpful way to look at it and depends heavily on what is even meant/understood by "best", including but not limited to your points on variety and how exclusivity for an actress would look. I also agree with >>42949779 on studios typically offering really different things so any apples-oranges comparison isn't going to be helpful anyway. That said, for the sake of discussion and if I HAD to choose, I would probably still make a case for Madonna. Obviously, any take on this is going to be subjective but I think an argument can be made pretty convincingly for them. Here are a few reasons why, in terms of the metrics you yourself provided:
Whether you want submissive hags, estrus hags, mean hags, humiliated hags, or reluctant hags in plots and sub-genres/fetishes ranging from incest, stepmothers, mothers-in-law, unsatisfied neighbourhood housewives, OL superiors, teachers, NTR, (gang)reipu, shibari, and beyond, Madonna's offerings cater to a wide spectrum of tastes and will probably have something you like in their bi-monthly release schedule - assuming you aren't vehemently opposed to the idea of mature women in the first place for some reason.
I feel like the common argument against Madonna is that "it's all the same!" and while I'd agree that they, as a whole, maybe rely on their existing body of ongoing flagship series more than other studios (rather than, say, pushing more unique "one-shot" releases), that oft-repeated sentiment fails to recognise just how varied these series are from one another. Sure, they are predictable once you have seen other entries in a given series but this isn't necessarily a bad thing and serious fans will still enjoy discerning the subtle differences between entries based on what different actresses uniquely bring to the table within the reliable and consistent parameters of a set-up they already know they'll like.
Another reflection of variety lies in Madonna's many clearly-defined sub-labels: MADOOOON!!!! (MDON-; candid, "out-of-character" gonzo), Monroe (ROE-; higher on the ripeness scale), Urekomi (URE-; doujin adaptations) and the recently added Achijo (ACHJ-; less drama, "pornier" scenarios). Even the mainline Madonna codes (currently JUQ-) often feature younger, up-and-coming or even leading freelance actresses such as Ichika Matsumoto and Non Ohana in more mature, drama-heavy roles that they wouldn't ordinarily be involved in. Therefore, in a hypothetical situation where one could only watch a purportedly "best studio", choosing Madonna means an immensely wide offering of potential scenarios presented, fetishes explored, and even actresses featured - albeit with an emphasis on the spectrum of mature women which, in itself, is vast and ties into why the The Hag Ripeness Classification System™ (patents pending) was developed in the first place.
There is undeniable prestige associated with Madonna as the largest and most glamorous mature-focused studio in AV so there's definitely no shame in being one of their exclusives. Their whole marketing shtick suggests an image of luxury all the while being a more accessible and mainstream gateway into appreciating mature women.
I've noticed that some of the larger studios like S1 and SOD seem to display heavy bias in how they promote their rosters, with a perhaps disproportionate (though not unjustified/unexpected) amount of the spotlight dedicated to only their biggest names. This leads to everyone else being relegated to being overshadowed by the big-name talent which perpetuates limited industry perception/acknowledgement as a whole, sometimes very unflatteringly as "filler" names. Madonna doesn't seem to play favourites in this way, though that might just be an outcome of being less (social) media-savvy as a whole. This could potentially mean exclusivity resulting in less cultural impact and/or industry influence overall for an actress irrespective of how Madonna actually treats/promotes them relative to other talent. Food for thought.
I know their pseudo-arthouse soft focus artistic lighting orange filter aesthetic isn't for everyone but for me, it's a perfect match with their melodramatic approach of exaggerated realism. Supplement this with their branding, consistent design language and tasteful, iconic covers and you get a powerful combination that makes following this studio very enjoyable and rewarding. Tastes can differ but what's on offer is nothing short of exceptional, consistent quality and polish.