Well, I've assumed for quite some time that Swords & Wizardry was the leader in retroclones and OSR games. Mostly this is because I just see much more activity around it than around any other system—I see more blogs talking about it, I see more products that reference it, etc.
But... according to the latest Orr Group Industry report (new ones come out quarterly), that's not true at all; by a large margin, Basic Fantasy (mostly a B/X clone, although with a few modifications) is the leader.
This also challenges my notion that the OSR is equal in general magnitude to the other schisms in the D&D hobby; although... well, I'm not certain that this sampling is representational. I believe, without necessary knowing this to be true, that many OSR games aren't broadcast the way some of these others are. In other words, OSR games are buddies who already know each other and game together, and they play their games without reporting anything to anyone. This is actually a much more old school paradigm about how to get your gaming group together anyway. Of course, if that's how they do it, then they won't register in the data that Orr gathers, because Orr gathers data based on people who are registered as Roll20 members, especially the "Seeks Group For" data.
But it is data, whether there's limitations, caveats and issues with it or not. Depending on the caveat, it's not necessarily better than no data (it could be actively misleading data) but I suspect that it's not entirely so. Maybe Basic Fantasy really is the gold standard for OSR games rather than Sword & Wizardry.
I kind of like the idea that B/X—my favorite actual older version of D&D and the only one that I'd seriously consider playing again for more than a novelty one-shot or something)—may be the one that is getting the most play in cloned form. Of course, I find it odd that retroclones of B/X would be so prolific (presumably mostly in pdf form) when you can get official pdfs of the actual B and X games for less than $5 each from Drive Thru RPG.
This is where I really part ways with the OSR: unlike the (apparent) taste of the average OSRian, I value elegance of design as much as I do simplicity and adherence to old school tone. Actually, probably more—although I am a fan of simplicity (a big fan) and I do admire old school tone. (I may describe that a bit differently than some, though—I like the DIY aesthetic more than strictly reproducing the actual old D&D tone. And I've never yet been very entertained by dungeoncrawls, nor do I expect that I ever will or could be.) All of these retroclones are more about reproducing the actual D&D rules in facsimile fashion than they are about "fixing" the rules for better elegance. d20 had elegance as a design goal—and it succeeded rather admirably, in my opinion—but they clearly didn't care for simplicity at all. An extremely brusque restructuring of d20 to recapture the "vibe" of the garage band publishing kind of feel of early TSR stuff, but with the elegance of more recent games then is my personal "sweet spot" and why I've been hyping m20 for years now. It's literally perfect for my needs; it's my holy grail of systems, if you will.
Y'know what I'd like to do one of these days? I have a few old school modules (actually; old modules)—B2, B4, X1, N1, U1-3, etc. that I'd have fun "translating" into FANTASY HACK and running. I think it'd be relatively little work to do the "translation" because the systems are simple enough (and, for that matter, relatively similar in most respects). Most of the ones I have kicking around are actually AD&D modules rather than D&D modules—but B2, B4 and X1 were all designed for B/X. In reality, that doesn't matter much; the systems are very similar, and I'd be translating them to m20 anyway.