Assuming Animal crossing sells more than 3.94M copies from Oct-Dec 2020, it will have crossed 30M in at most, 9 months 2 weeks (call it 10 months).
PUBG hit 50M in 16 months on the market. It probably crossed 30M in its initial summer/fall 2017 frenzy so I bet it counts.
I don't know if we have a definitive answer on any other title doing it faster aside from GTAV, which did it by Dec 2013 (~3.5 months).
Yearly COD never gives specific confirmed figures as far as I know (just canned "biggest $$$ opening ever" type of statements usually (someone correct me if this is wrong).
Minecraft probably did not do it, that's just a product of selling 15-20M yearly. It may have had a year where it moved 30M units, but the launch->30M timeframe was likely not shorter than ACNH.
PUBG hit 50M fast enough that 30M makes sense, but don't
RDR2 crossed 30M in the quarter of Jan - Mar 2020, so that took 15-18 months. AC will blow past that.
There aren't very many exclusives even above 30M, let alone an exclusive that moved 30M in under a year. MK8D/PkMn RGB, Wii Sports/Fit series, NSMB - none of those sold anywhere near that fast, they took years to climb up there.Last edited by mk7sx - on 19 November 2020