I mean, other than a few of us, most PDF companies keep their business info private.
We can see reviews and such, but all that does is tell us an opinion on the quality of the product, not how well it sells.
"Bench marking" is a business term that means comparing your business to that of your competitors. All well run companies do it, and most don't get to see their competitor's books.
Bench marking includes:
1. Buying your competitor’s products, using them and evaluating them against your own products.
2. Search industry media, looking for your competitor’s advertisements to get an idea of their marketing strategy and budget. Compare that estimate to your own.
3. Comparing your prices and quantity of material to that of your competitor.
4. Check RPGnow top lists to see where your competitor’s products re ranked. Compare that to your own rankings.
5. visit their web site.
6. Read their product schedules and press releases.
7. Analyze their product mix. Again, compare that to your own.
If you mean most popular (and I don't think this directly equates to sales... It helps, but it's not a direct correlation), then it would have to be EN Publishing.
Let's get right to it, they have the forum that everyone comes to...
EN World is a fine company to benchmark against, but they are far from the only company in this industry. And they are not the only ones doing innovative things.
The best thing to do is to keep your eye on most of the major players and a few of the promising smaller players. The problem is, there are so many of us, it is hard to do that. My attention usually roams from one to another every couple of weeks or so.