Easy question, difficult answer. Which one of these 3 heavy-hitters will have sold the most copies when all is said and done? Will it be Mario Kart Wii, Mario Kart 8 Deluxe or Animal Crossing: New Horizons?

What each has going for themselves:

• Mario Kart Wii: Huge lead that needs to be overcome first, and still has new shipments to this day in practically every quarterly report, though no big ones. Still, this can amount to an additional million or so the other games need to reach higher than the current sales. Current shipments: 37.32M
• Mario Kart 8 Deluxe: Insane legs since it's launch in 2017. Back then, people didn't even believe it could catch up to MKWii even with the Wii U version added on top, these days it's possible it can do it without. Current shipment: 26.74M
• Animal Crossing: New Horizons: Just sells like crazy during this pandemic, already having become the second-bestselling title of the Switch in less than 4 months and with no sings of slowing down anytime soon. Current shipment: 22.40M

Animal Crossing New Horizons
But I also see MK8 crossing 40 million units

Well I've predicted that MK8DX will hit 50 mil and while it's certainly not impossible I don't see AC going quite that high.
AC will probably overtake it in sales by the end of the year, but I expect MK8DX to retake the lead somewhere far down the line.

40 million is fairly doable, but 50 million?

Current attachment rate for MK8 is 42% and it's decreasing, not increasing. For the last 4 quarters the attachment rate for MK8 was 36%, looking to this trend I'm guessing something more like 32-34% for the next years. For MK8 sell 50 million with a 32-34% attachment rate Switch will need to sell from 147 to 156 million copies

And I don't think it's a realistic prediction for Switch

The other possibility is MK8 having crazy legs, about 1.5 million for each quarter in the next 4 years even in the years of Switch's death and finish its run with about 40% of attachment hate

If anything, MK8 will end over 40 million, but behind 50 million 

I'm guessing MK8DX. The last Animal Crossing sold half of its copies after 2013, but I'm not sure if New Horizons will have long term sales quite as good, if only because it is being bought by everyone at once. MK8 Deluxe may have trouble reaching MKWii levels if attach rates continue to fall, but we have to remember that MK8DX still hasn't been bundled with the Switch, whereas MKWii was from 2011 onwards, selling at least a few million copies this way. So I expect MK8DX to reach 40 million, though not 50 million.

That said, if Animal Crossing reaches 40 million copies, I wouldn't be too surprised, especially if the game gets continuous support. That game is ridiculous.

Regardless of whether its due to higher Switch sales in general fact is that MK8 shipments have been increasing with each year:

2017: 7,33 mil (launched in april)
2018: 7,69 mil
2019: 7,94 mil

And I fully expect that to happen again this year since its currently at 3,78 mil after 2 quarters compared to 2,87 mil after 2 quarters last year.

That puts it at 31 mil or more at the end of this year. I expect the Switch to have almost 4 years of lifespan and at least 60 mil sales left in it after that. 19 more mil for MK8 seems quite achievable to me given that. This is all assuming ofc that MK9 isn't happening on Switch.

I still think Mario Kart Wii. It might be very close though. Though maybe I'll change my mind after we see what Animal Crossing ships holiday quarter this year.

Both games will cross 40m. Although I expect AC to outsell Mario Kart 8 Deluxe initially, I think AC will eventually re-lose ground to MK8D and lose to it by hair. Thinking 42 vs 41m.

My money is with Animal Crossing, especially if they prolong its life with seasonal content updates for years to come. If they do a full blown paid expansion pack at any point that goes beyond the current scope of the game now or the updates we have received thus far, I don't see it stopping even remotely soon enough to not be close.

I'm still split between MK8DX and AC:NH, but I'm confident both will beat MKWii, something pretty much nobody thought even possible when either of the other two games released..