Worst one I've played? Easy - it was one of my sister's, called Action Girlz Racing. Yuck, even the title makes me hate it.
Made by the same lot who did the (un)sensational Ninjabread Man, you start the game with 1 track. Just 1. There are no differences in unlockable characters or karts. The graphics are godawful, the 1 music track in the whole game is completely lifeless, the races are ridiculously slow-paced, the controls are incredibly unresponsive and the hit detection is abysmal (sometimes you ram something and bump off, other times you'll be sent flying 180 degrees through the air.)
It has literally no redeeming features, just the poorest Mario kart rip off and indeed, worst game, I've ever played.
My Youtube career kinda ended, so...I really have nothing to put here anymore...