If you mean powerful as in 'big explosion', than it has to be the blue shell. If you mean powerful as in harder to dodge, it is the lightning.
The labor union's most powerful weapon is the strike.
the most powerful weapon is the lost katana it is +110
the most powerful weapon is the supernova lvl 99. that is the mega version of the harbinger
The weapon of the King of the Gods, Zeus, had to be the most powerful. Otherwise, he would be overthrown by the other gods. Thus, it was his thunderbolt that was the most frightening weapon.
I believe it is mario kart 7... not even kidding. Out for 3ds but to answer yours it would be II.
For the karts the Wild Wing. For the bikes the Mach Bike.
a nuke
canyon
yoshi valley
Wii fit. why? Mario kart has the cool wheel, but the wii fit has the COOLER baord -- which can be used for most games. mkwii rules!!!
the cannons
A thermonuclear warhead