Since this thread is solely about system selling power:
1. Mario Kart: Obviously the biggest, being the best selling Nintendo game on the four most recent Nintendo systems by a comfortable margin.
2. Pokémon: Near-yearly releases and sub-par performance, but still consistent sales.
3. Animal Crossing: Obviously inflated due to the pandemic, but ACNH still would've been huge and likely still the third best selling Switch game. WW & NL were also huge.
4. Smash Bros.: Consistently near the top of the pack, and it only gets bigger with the buzz around who's joining next.
5. 3D Mario: These games usually showcase new hardware the best. Mario Odyssey was actually briefly the best selling Switch game when it first launched.
6. 3D Zelda: The open world aspect has really caused sales to explode and while I am listing it below 3D Mario despite it doing better on Switch 1, I still feel 3D Mario will more consistently outshine it going forward, at least as far as system sellers goes.
7. Mario Party: Always been a consistent seller, but those yearly releases in the N64 & GCN days definitely hurt the chances back then. We've seen now how consistent the sales are and how big it is for Nintendo.
If I was asked this question during the DS/Wii days, I would've included 2D Mario. Obviously other franchises like Splatoon, Kirby and Pikmin are big but not quite system seller caliber.