Update: Thread's title has been changed to reflect all possible first party titles instead of the original 3.

Pokemon

Best selling instalment: Red/Green/Blue at 32m

Imo: Would be a mountain of a task to pull off but with the way the Switch is performing and the upcoming title essentially being the first mainline instalment ever made at a console level it's just as much of a possibility as it is difficult to achieve after all SMO is essentially a return of the SM64 approach many have wanted a return of and it moved 9m in 2 months, my current bold prediction for the game is that it will outsell Gold/Silver though so my personal verdict is that the series has seen an uphill climb in recent years and even delivering a competent instalment on the Switch could see it possibly happen but some other factors have to align, the game will be a monster but how much of a monster remains up in the air.

Animal Crossing

Best selling instalment: Wild World at 12m

Imo: I'm going to say yes on this one AC averages 10m and even the last instalment on 3DS may eventually pass WW as it stands now (NL has sold 11.5m), I can see the NS gaming hitting the standard 10m no problem and eventually outselling WW or NL if the latter becomes the best selling overtime with the sales obtain through legs.

Metroid Prime 4

Best selling instalment: Prime at 2.8m

Imo: The trickiest one to predict as this one comes down to how much exposure it's given and the approach taken, the original Prime itself had a luck of the draw as it got exposure albeit through controversy but at the time people didn't know what type of game it was and many bought it thinking it was going to be a Halo type shooter, this was the era before the likes of Youtube and social media where you can look up gameplay plus many forums and such yourself all people had back then were screens in a magazine.

Prime 4 could beat out the original though given Switch's performance, it's pretty much confirmed it's co-developed by Namco and a new internal team consisting of the people who were behind the cancelled Star Wars 1313 so the approach of the game may vary compared to the Turok inspired template Retro (who themselves were the former Turok team) brought us. I'll go for yes here as if Xenoblade 2 can break its own barrier then Metroid can as well.

So Far:

Legend of Zelda: Breath of the Wild - 10.28m (Switch version only, total with Wii U version is around 12m)

Super Mario Odyssey - 12.17m

Xenoblade Chronicles 2 - 1.53m

Splatoon 2 - 7.47m

Pokemon and Animal Crossing can absolutely become the best selling games in their franchise, Metroid Prime depends on how well Nintendo can keep the console healthy

I think Prime will take the cake if it's reasonably well received. As long as it isn't generally hated I think this is a shoe in honestly.

Animal Crossing would almost be a guarantee for me as well.

Pokemon is iffy as you said. Beating RGB would be a crazy feat but being on a home console gives it a chance for sure. They really want the 2018 release date for this so that is can sell for the majority of the consoles lifetime.
I don't quite think it will reach it but I think between 25M-30M would be legit.

animal crossing I think is a definite yes as I think it will do comparable in japan but much better in us/eu . Plus I am excited for it will be my first animal crossing since the GC game

Let's hope all of them become the fastest selling titles in their franchise.

I disagree with your statement regarding people who bought Prime because they thought it would be Halo. People bought Prime (and even Gamecubes) because it was Metroid. The fact that it was a GOTY title and had consistent 10/10 scores (and yes, the internet existed then) also helped take this title to the highest charted position for a Metroid title.

And if Prime 4 is anywhere near as good as any of the three Prime titles it will easily sell 3 million, especially if it has a multiplayer battle mode like Echoes.

Pokemon will have a problem in this regard. Previous mainline Pokemon titles have retailed at regular handheld price (around 39.99$), but this will almost certainly be priced at full price, 59.99$. That is a pretty distinctive gap there. And the fact that the first installment sold more than 30m copies. There's nothing short of another pokemania around the world (or a reeeeeally big hit in Chine) that can pull off this numbers. Other than that, it will depend on how good the 8th gen is.

Animal Crossing is a 50/50. I don't really see it doing that much, but I don't know or care that much about the series anyway.

Metroid Prime will beat Prime by very little, but that's it. 2.8m copies is not that big of a landmark for 1st party Nintendo titles.

Yes it can.

Why?

Because resolution matters:

It ll be the Monster Hunter Effect... It wont be held back, gameplay or graphically, by the 3DS anymore.
So it ll be the best Pokemon in forever, and will sell well (if the developers arnt lazy about it).

Metroid Prime 4 & Animal Crossing = im not as sure off.

Animal Crossing & Metroid, possible

Pokemon, not likely

No to Pokemon

Yes to Animal Crossing

And Maybe to Metroid.

Pokemon will never reach its Gen 1 peak again, Animal Crossing should have no problem beating New Leaf, while MP4 is harder to predict as the brand seems damaged but if anything can fix it, it will be the Switch.