Vs Metaknight
Spam Fireballs. Metaknight can't handle projectiles very well, so this'll definitely annoying him and rack up some decent damage. Try to get Metaknight off the ground, preferably with Mario's utilt (since it rocks). From there, just start throwing out bairs (think WOP), its range and priority is better the most (if not all) of Metaknight's moves. Mario has more range then Metaknight (never thought I'd ever say Mario has more range then anyone), so try keep that spacing tight with ftilts, bairs, and dsmashs.
When it comes to KOing Metaknight, use Marios usmash (try hyphen smashing it, that'll catch him by surprise more then you expect it too). Dsmash and fsmash work too , with dsmash being easier to land and fsmash being a lot stronger. Other decent killing options are nair, bair, fair(spike or doc punch), and even uair surprisingly.
edit: stupid typo v_v