I agree with the people saying that we should try to not be so biased towards Nintendo, but an exception needs to be made for Mario Kart. The series is insanely iconic, it might as well be synonymous with the genre for many. That begs the question, though, which game? To start off, here are the games in the series I don't think to deserve to be in this collection
Super Mario Kart (yeah, it started the series and all, but it didn't age well and few are as nostalgic for it as other games in the series)
Super Circuit (undeniably the most forgettable main series game in the series)
Double Dash (I feel like I'll get some flak for this one, but hear me out. DD is a good game, but it's not much more than that. It didn't revolutionize the series, most of its new features were one offs, and it doesn't have the same afterlife as a certain other GameCube multiplayer game. I just think it would be a bit misplaced in this collection)
Seven (Same situation. 7 is actually one of my favorites in the series, is it one of the 100 most important games of all time? No.)
Tour (I don't think this one needs explained)
Arcade 1/2/Home Circuit (did you remember these ones existed?)
I think you could make a case for every other Mario Kart game, but one stands above the rest - 8 Deluxe. By the time the Booster Course Pass is done, it will have 96 tracks - triple the number in DS and Wii, and six times the 64 course list. That alone gives it a good argument for being the best in the series, and it doesn't hurt and it also has incredible graphics, an insane soundtrack, great mechanics, existent online, brand new quality of life features ( such as the item select) and... a serviceable roster and battle mode.