Current Standings:

Rough and tough forecast:

47-52M for MK8D and 50-55M for ACNH.  We've seen GTA and MC persist on with basically no drop in sales for years on end, so maybe those MK8D drops are a little harsh if NSW is staying above 15M until 2022.  Also for ACNH I pegged this year at 4.2M for the current quarter and 5.2M for the holiday quarter.

Now that I think about it, ACNH in Japan in 4 weeks has already done 332K retail (which means ~500K w/digital @30%) which would put quarter on pace for 1,100K (1,575K w/digital) which equates out to closer to 5M WW shipped and there's little chance that the holiday quarter is lower than Jul-Sep IMO, so maybe it goes higher than 32M for this current year.

I would bank on ACNH just because of the insane looking pace so far, but MK is a leggy monster so who knows.

I didn't think anything could be Mario Kart (except maybe Wii Sports, but Nintendo isn't making that anymore).  Then Animal Crossing comes along and destroys Japan and the rest of the world.  It's like Godzilla in a video game form.  Animal Crossing has almost caught up to MK8 and it hasn't even had its first holiday yet.  In the end I think it will be

AN:NH
MK 8 D
MK Wii

Neither of my top 2 have sales that are going to fall off a cliff, just like Switch sales aren't going to fall off a cliff.  Be careful of anyone trying to sell the snake oil that MK Wii will end up on top.  They are just wishfully thinking that the Switch will stop selling magically even though it has two games on track to do 40m+ and several more within spitting distance of 20m.

It's been almost a year, time for an update of the current shipments:

MKWii:   37.38M
MK8DX: 35.39M
AC:NH:  32.63M

Looks like Both Switch games are gaining very fast on the Wii title. It's possible that by the end of the year, both would have passed Mario Kart Wii. And to think many thought such a feat was impossible about 2 years ago...

It's possible that MK8DX outsells MK Wii this quarter.

Since ACNH has already slowed down I don't see it outselling MK8DX, which is going to cross the 40mil mark this year and end up north of 50mil as the best selling Mario game ever. It could go over 60mil, but that depends on if Nintendo decides to release MK9 on the Switch or not.

ACNH is definitely going to sell over 40mil, but not necessarily this year and 50mil is possible but I don't see it as guaranteed anymore.

ACNH & MK8DX are well on their way to surpassing MK Wii this year. I don't see ACNH surpassing MK8DX though.

Ultimately, depending on how long it takes until a Switch successor releases (hoping for Holiday 2025), I could actually see MK8DX reaching sales north of 70 million. Possibly even surpassing Wii Sports (at least if you include MK8 on Wii U). I could see ACNH reaching about 60 million in its lifetime.

They'll fall off pretty quickly once the Switch successor gets its own MK & AC, which I could see happening within the first year of the successor.