Another damn banana?!
Green Shell
The GIF makes it look like a badass item, but IT'S NOT. This thing is 99% useless, because it has no drive. (You know, like that 34-year-old guy living in his parents' basement). The shell just goes willy-nilly and only if you're really lucky will it actually hit someone.
Nintendo / Via rebloggy.com
Triple Green Shells
This item is better than the single green shell, mostly because you have a slightly higher chance of hitting another player. But your plan will probably backfire.
Mushroom Boost
This item is about as exhilarating as this photo. It gives you a quick boost, which ONLY matters if you're neck and neck at the finish line with that cute girl from down the street. Sure, you may win, but she's not gonna be back to play again anytime soon.
Nintendo / Via mario.wikia.com
Boo
Yes, Boo was originally an item before the Nintendo Gods let him be a racer. He makes you transparent, but only for a short time. Sounds cool, right? WRONG. The item allows bananas and shells to pass right through you, but you're usually never near any while using it. How convenient.
Nintendo / Via random-ness.wikia.com