Brendan,
1700+ posts and you still haven't learned the answer to that question?
The short answer is: Among end users, there are many who favor S30V, others who favor BG42. But the differences tend to be minor (e.g., which polishes better, which is a little easier to sharpen, which can "I" put a better edge on), and just about everyone regards them both as the best all-round "stainless" steels. Objectively, CRK treats S30V to a slightly lower RC than it did with BG42. Why? I don't know, but it remains a "heated" topic.
Like nearly every other steel out there, BG42 is a steel originally designed for applications other than for use in cutlery; in this case, as bearings capable of achieving high wear and corrosion resistance, IIRC. While a few custom makers had used BG42 previously, CRK was the first manufacturer to adopt it; and at that time, it was considered a fairly significant upgrade from the original ATS-34 blades. But BG42 is apparently quite difficult to work at the production end, and consequently among production knives it was found almost exclusively on the Sebenza (big, small) and Umfaan. A few other companies have since started using BG42, but with mixed results. CRK, it is generally considered, got the heat treatment right, and better.
S30V is, I believe, the first American steel designed specifically as a cutlery steel. Chris Reeve had a hand in its development by Crucible, with the aim of creating a highly refined steel with an optimal balance of wear resistance and toughness in a "stainless" steel. It came to market a little less than 3 years ago, and has been widely adopted by custom makers and higher-end manufacturers since -- due to its high performance, but also because it is relatively affordable (relative to its performance) and presents no unusual challenges in production.
I doubt many would call S30V the "perfect steel," at least not outside a particular context; but most makers and users believe it is the most balanced, all-round stainless steel currently available.