Best kart and character combo in Mario Kart World?

There aren't separate parts to mix any more, you pick a character and a whole vehicle, so it's simpler than 8 Deluxe. Heavier characters (Bowser, DK, Wario) have the higher top speed but take longer to get back up to it, lighter ones accelerate and turn quicker. With 24 racers you get bumped and hit constantly, so a lot of people end up preferring a middle weight with decent acceleration over a pure speed build. Honestly the gaps are small. Pick something whose handling you like and drive it for a week before you decide it's wrong.