Not sure about the best, but my favourite is Rhythm Heaven Fever.

If Yoshi’s Island is counted as a separate game (even though the original is called SMW2), I’d vote that. Otherwise, Star Fox 64.

Pikmin 1 or LM3, both are acceptable answers.

The two fire emblem games featuring Marcus, Bartre, Melinus and Karel on GBA. Blazing Blade and Binding Blade. I go back and forth between which game I enjoy the most.
At a close third place I have Advance Wars Dual Strike. I simply enjoy turn based strategy I guess.

Banjo & Kazooie, Starfox64, DK Tropical Freeze, Pikmin4 are my faves outside the top3 franchises.

Xenoblade is first party.

There are a lot of good candidates, Star Fox 64, Donkey Kong Country 2, Donkey Kong Country Tropical Freeze, Pokémon HeartGold/SoulSilver, F-Zero GX...

I'd probably pick Tropical Freeze I think.

This.

If not, then Donkey Kong Country.

Assuming “Mario” means the Super Mario platform games, I think the best eligible titles are:

1) Xenoblade Chronicles
2) Mario Kart 8 Deluxe
3) Banjo-Kazooie

it depends form the offering...

Of course Animal crossing are great fun games...

For me personally I go for Pikmin, Kid Icarus and Kirby,