I'd be very surprised to learn that Creature Collection wasn't the best selling non-WotC d20 product. I'd suspect that it's probably one of the best selling non-WotC rpg products of the last few years.
It wasn't just early. It was the:
First non-WotC d20 hardcover.
First d20 monster book (WotC or non-WotC).
First non-WotC d20 "core" type book of any kind.
I'll agree with Chris and James that being early meant a lot in the d20 industry. While I'm not referring to their products, you could literally put the d20 logo on any old piece of crap in those early days and sell 10K to 20K. (And even some of the good stuff from those days hasn't aged particularly well.)
I won't agree that it's impossible to see sales figures like that today. It's just harder--which is OK with me, actually. Malhavoc products are selling very well and I suspect that, while not the best selling d20 product of all time, Arcana Unearthed is likely to be right up there, and in fact is still selling very strong. I'm very proud of that. The fact that it sold so well in a period when it seemed everyone else in the industry was talking doom and gloom made it particularly interesting time.
Of course, a big part of the issue of sales is distribution. It's probably a bigger factor than individual product quality (although consistent quality might get you better distribution, so it's muddy). In any event, it would be a mistake to think of the game industry, or the d20 segment of it, as a level playing field. There's a world of difference in sales of a product that regularly gets into bookstores (as well as all gamestores) with one or more full-time salespeople working to get it into distributors' and retailers' hands and a product that isn't even regularly stocked in every game store.
I suspect, as things move along, this difference will become more pronounced, not less. More and more distributors, let alone retailers, are choosing to carry only a few d20 companies' products and don't even look at new ones.
This means, of course, that the occasional gem slips through the fingers of the system, like last year's Magical Medieval Societies.