First place for me is a tie between the Golden shroom and the Star. The golden shroom gives you infinite boosts, which allows you to take as many shortcuts as you want, and the star speeds up your cart, makes it invunerable, and allows you to bash into your slower enemies. The golden shroom is more common, though.
Second place goes to Bullet Bill. It turns you as fast as a speeding bullet (Ow, bad pun. >_<) and runs you into opponets, damaging them.
Third is the scourge of 1st place hoggers everywhere, The spiny shell. There is nothing that can stop it from ruining your race if you are in 1st, and, if 2nd or 3rd are close enough, the explosion will hit them too.
The most overrated item is lighting, all it does is makes the karts smaller and allow you to run over them. It also makes them slightly slower. Big deal.
The worst item HAS to be the blooper. all it does is spay ink on the top screen, and all you need to do then is drive using the map screen. (It works, trust me.)