I'm a competitive Mario Kart 8 player, and with a lot of experience growing up, I would say that the most annoying item (and the worst) is the Thunder Cloud.
This item is only good for maybe taking a shortcut and gleaning from it's evanescent speed boost. Otherwise, if passed to you, it can be an automatic race killer. Not only is it fairly common, it's hard to pass to others by bumping into them because of Mario Kart's infamous lag. I can't fathom how nintendo thought it would be balanced to make an item that shocks a single player alone, especially considering how good the shock item itself is.
Luckily, it was only terrorizing players in Mario Kart Wii, which is arguably one of the best and most balanced Mario Kart game of all time.