who would you use against,
Dedede
Metaknight
Mario
Bowser
I'm gonna assume you were addressing me and meant Mewtwo or Marth.
Dedede: Mewtwo, for sure. D3 is super hard for Marth to kill, with his fast fall speed, heavy weight and crazy long recovery. Also, his gimp game on Marth is strong. Mewtwo has it easier.
Meta Knight (disclaimer: I haven't fought a good MK since 3.02, but back then, Mewtwo gave him a hard time) I'd say Mewtwo. Hopefully Emu can back me up or correct me on this one.
Mario: I prefer Marth here. You're fast enough to outmaneuver him, can swat away his fireballs with jab/fair, and your disjointed sword lets you zone him with dtilt and double sh fair. Sweetspot the ledge though or you're gonna get caped.
Bowser: Marth poops on Bowser in my experience. Get him off the ledge and dtilt and laugh. If you're wary of Bowser's armor and surprisingly long range, your grab game can foil his crouch shenanigans and lead to big punishes.