Juggle, they can't do anything against a good juggle without good aerial mobility, a good teleport or a ton of jumps. Those who have those become more match-up specific.
Samus is stupid easy to juggle though, up-air/tilt beats all her downwards options and she can't dance or teleport like a Zelda or a MewTwo can.
For characters with those long omni-directional teleports (Zelda, MewTwo), it turns into a game of reads and beating them in neutral. I find MewTwo to be a problem though, you should ask someone else.
Jigglypuff is like melee, just beat her with your range, f-air's good, f-tilt's good. Put her into a protracted edge-guard then kill.
Metaknight's a pain, but Marth has good kill set-ups anyway, so juggles are less important. I think f-throw to f-smash is consistent.
Kirby has some interesting options, but so long as you can deal with the 2 cutter variants you should be good, just act like he's Jiggs, and make for a long edgeguard.
Dedede is easy to combo because you're not going to miss him. Put him on the back foot and spike or edgeguard. Watch for the disjoint though, especially f-tilt.
Peach is juggle fodder, just stay under her so long as she has no turnip. In neutral d-tilt is your friend, and I think f-throw to (wave) f-smash connects for a good while.
This is how I do it, but admittedly neither my friends nor I are really good. Corrections would be appreciated.