Marc Newman of HouseofComics.com asked whether Detective might not have been the top seller prior to the launch of Action Comics. The challenge here is that in those days, National Periodical Publications (the precursor to DC) sold ads in the "Detective Comics Group," not in individual titles — and so individual title sales are not known.
However, we can see from Maheras' ABC research that in January 1939, all the issues on sale from the Detective Comics Group had combined sales of 709,379 copies. That group in that month for the ABC included...
• More Fun Comics
• Action Comics
• Adventure Comics
...and any issues that shipped that month would combine for the 709,379. Now, we can't really link up specific issue numbers to these titles, because the cover months and the auditing months might not be the same. But we know that at this time, all four books were monthly. So we're looking at a four-way split.
Now compare that with Famous Funnies, which alone reported 357,386 copies sold in January of 1939 (and close to it in the months on either side). That, and some of what we see in some other books suggests that the kids' titles and longer-established strip reprint books were more heavily circulated than the adventure titles before 1938. So my suspicion is that Detective probably still wasn't #1 in that stage. Adventure comics do come more into vogue in this period — More Fun's last humor cover was in early 1938 — but thereafter I would venture that the combination of Action and Superman likely preclude chart leadership by Detective (or, for that matter, Batman, until the 1960s). That's not even getting into the Dell Disney books, which entered 1960 in the top slot.
We're in a realm there where there are some key books where figures will never be known even from the Audit Bureau, but I think this is a decent guess.
Incidentally, you may ask, what issue of Detective Comics came out in January 1939, for which the numbers above report? The January 1939 cover-dated issue was Detective #23, but with cover post-dating going on, I suspect it well could have been Detective #27 (above), featuring the first appearance of Batman. (I imagine that's why Maheras picked that month to research!) So from this, we get a fair idea of the starting population of that issue: likely lower than the starting populations of most key Silver Age titles....