Bowser's the name, speed's my game!
[spoiler]I don't recall a game where Yoshi's weight class was OP unless it was pre-Double Dash. In Mario Kart Wii, Funky Kong was the most overpowered character on the Flame Runner/Bowser Bike in any game ever, while Daisy was somewhat broken with great speed and the best drifting in the game on the Mach Bike. In Mario Kart 7, it was Metal Mario's weight class with monster tires on the B-Dasher that consumed the pro players. I wouldn't say Mario Kart 8 has any 'broken' characters, but the best would definitely be Bowser/Wario/Morton on the +0.75 speed karts with Cyber Slick tires, but it takes a while to get used to the outside drifting[/spoiler]