It really depends on the game really. In Mario Kart Wii, the Blue Shell is as much of a terror as people say it is. In Mario Kart 8, my most feared item is the Green Shells just because of how the AI either aims you and/or even richochets them just where they need to go or are jerks that throw it behind them. There's no warning when a green shell is coming to it can come out of nowhere and screw up your race. It's why whenever I play 150cc I hold on to any protecting items for as long as I can, cause even in my attempts to beat Egg and Hyrule Cup in 150cc (Which I did for each on the 1st try luckily, didn't get that rusty even if I haven't played in a while in favor of Smash Bros. for the 3DS) there was at least one time in each cup I would of been blindsided by a Green shell that I was protected from.