The audiobook of 'The President Is Missing' read by Dennis Quaid (and others) was a whole production. That kind of star power in the narration draws in listeners who might otherwise skip it. It's another example of how a book's popularity can be amplified by its presentation in other media formats.
Forget series versus standalones. The true top-selling 'book' might be one of the early 'Alex Cross' novels that has been in print for 30 years and sold steadily every single week since publication. 'Kiss the Girls' or 'Along Came a Spider' have a cumulative sales tally that a new release, even a huge one, can't match in a single year. The long-tail sales of those foundational books are monstrous. So while we debate which new release hits #1, the all-time champion is probably sitting quietly in the backlist, selling thousands of copies a week to new readers discovering Patterson for the first time.
This might be a hot take, but the later Alex Cross novels don't rank as high for me as the originals. The early books had a grittier, more visceral feel, almost like horror-thrillers. The newer ones, while still page-turners, can feel a bit more procedural and reliant on the established formula. For my personal top-tier Patterson thrillers, I'm looking at 'Kiss the Girls', 'Pop Goes the Weasel', and 'Roses Are Red'. They captured Cross at his most driven and the villains at their most chilling. That era is the peak of Patterson's crime writing for my money.
I'd be more interested in seeing the sales data plotted on a graph over time. There would be huge spikes for the first book in each new series, a spike for the Clinton collaboration, and maybe a gradual decline in per-title sales for the later series entries as the market gets saturated. But the overall area under the curve—the total volume—is what's truly staggering. He's sold a mountain of books, and arguing over which peak is tallest misses the scale of the mountain range.