Worst Mario Kart: Either Double Dash (Gamecube) or 7 (3DS)Wii had the worst Mario Kart, the worst Zelda (Skyward Sword), the worst Wario Ware, the worst Metroid (Other M), the worst Fire Emblem (Radiant Dawn), the worst Smash (Brawl)... It was a Trojan horse to sneak bowling into nursing homes.
Worst Zelda: Triforce Heroes (3DS)
Worst Metroid: Federation Force (3DS)
Worst Fire Emblem: Fates (3DS). Even Conquest by itself is worse than every other Fire Emblem bar the other Fates routes and maybe Shadow Dragon.
I'll give you the worst Smash, however.
The Wii could also play the best Mario (Galaxy), one of the best Zeldas (Twilight Princess), the best Fire Emblem (Path of Radiance), and the only good Xenoblade.