The way I see it is Mario Kart Wii was where modern Mario Kart began. While the series has never been a slouch (maybe excluding Cube and Super Circuit), I don’t think 7 or 8 are worse, they just weren’t the monumental leap in quality over previous iterations. The bar was set very high on the Wii, but IMO Mario Kart 7 was an improvement even if just for having a proper modern handheld version. The DS game was great, but it’s like the N64 version, there were issues with it that were forgiven because there was nothing superior to compare it to. Mario Kart 7 didn’t properly succeed Mario Kart Wii, but it did act as a great companion game, and Mario Kart 8D took the benefits of each of those games and improved on those.
I think Wii still does some things better than the Switch (IR aiming beats gyro, Wii’s party games are still king, and the best Super Mario console is still the Wii). Switch, IMO, in spite of the monumental fuck up with the premium subscription service, has surpassed the Wii in catalogue collections. While there’s no Chrono Trigger and FF6, or Ogre Battle 64, the Switch now has FF7-X, Grandia 1 and 2, FF Legend 1-3, Mana games, multiple ways to get Sega’s catalogue, Langrisser, Romancing Saga games, Dragon Quest games, and a whole lot of Tales games. I think FF6, Chrono Trigger, and the rest will come someway, somehow. I’m hoping Nintendo’s new and much more advanced gyro tech catches up in precision and quality to their primitive IR and accelerometer tech - they’ve been trying to fix that issue that wasn’t really broken for 10 years now.
Anyway, Nintendo’s undisputed champion is Breath of the Wild. The best shot of any company at currently surpassing it is probably Breath of the Wild 2.