Attacks That Out Prioritize the Tornado
This isn't every move that wins against MK's B. I only listed the ones that are the most useful to the character.
Use these moves when Metaknight comes at you from the side on the ground. If you have this move, don't shield, attack!
NOTE: Apparently Metaknight's Tornado GAINS priority if you mash the B button, and becomes weak again towards the end. This list is tested against the middle timing of the tornado, where it's strongest. Bolded attacks are the ones that are extremely useful.
Bowser – F-tilt
Captain Falcon – Down-B
Charzard – F-tilt, F-smash
Diddy Kong – F-tilt
Donkey Kong – F-smash, F-tilt
Falco – B, down-B
Fox – F-smash
Game & Watch – Dash attack, D-smash
Ganondorf – Down-B, F-smash
Ice Climbers – Down-B
Ike – F-air, down-B
Ivysaur – F-smash
Jigglypuff – Side-B
King Dedede – D-smash
Kirby – F-smash
Link – Down-B, up-B, F-smash
Lucario – Down-B, B charged
Lucas – F-smash, side-B
Luigi – Nothing
Mario – D-air
Marth – F-smash, forward-B, down-B
Metaknight – D-smash, F/B-air
Ness – F-smash, F-air, side-B
Peach – B, F-tilt, F-air
Pikachu – F-smash
Pikmin & Olimar – Up-B
Pit – F-tilt, D-smash
Rob – B, N-air
Samus – Z-air, Super Missile
Sheik – B, N-air
Snake – D-tilt, B
Sonic – F-smash
Squirtle – Side-B, F-smash
Toon Link – Down-B, up-B
Wario – F-tilt, F-smash
Wolf – B
Yoshi – Up-B
Zelda – Side-B (mid-distance)
Zero Suit Samus – N-air
all of this from
http://smashboards.com/showthread.php?t=161939&highlight=mach+tornado