Was wondering that too. Originally thought it might have been something in the 90s (only that I realized the 2000s are what are 20 years ago now )
Think it'd probably be the first issue of the Heinberg relaunch, it ranked #4 in in the month it came out at over 130,000. Largely coming off the momentum of Infinite Crisis.
Bit on the demographic makeup of WW's readership I found interesting. Would be interested to see the makeup of other books.