I'm one of those crazies who enjoys min-maxing Mario Kart, but I gotta say I don't really like having to build my vehicle in this game. It really just slows down the game and makes it take so much longer to get into a race. My friends and I always spend like 5 minutes in the vehicle selection screen agonizing over what we want to pick. It doesn't help that there's no easy way to compare two parts if they're distant from each other on the list.
And believe it or not, even in Mario Kart Wii, there was a "best build" that the community agreed upon. In that game, characters gave ever so slight boosts to different stats, and Funky Kong gave a small boost to speed, so the community generally agreed that he was the best character. But in reality it didn't matter that much. I played King Boo because I liked him more and I would almost always come in first in online races, even against Funky.
It's the same in MK8, where the community has pretty much agreed on a "best build" at this point, but (to a certain point) that doesn't matter that much. The also nerfed bikes, which were strictly superior in MKWii, so it seems that you really can't go wrong with vehicle choice in this game.
Of course, if you're good enough at the game that you are able to drift around every turn and are going top speed the entire time, then a vehicle with higher top speed and less handling will benefit you more. That's really how "best builds" are decided upon: if it has high top speed, it's better (in a nutshell). But Jonny's absolutely right. If you can't take turns correctly, then you'll just fall off or go off-road, and that top speed isn't helping you at all.
There's a hell of a lot more to it than that, but I wouldn't want to bore anyone with the details.