Metaknight is very difficult, he's the best character in the game, has great options but we can always punish mistakes.
Great tools for this match-up is Fireball's: Metaknight has no projectile so in a way we CAN force him to approach in ways that aren't that safe for him. Some Metaknight's will Tornado as soon as you start Fireball'ing take advantage of that (I believe upB will always beat a Tornado if he's in on your shield it has invincibility frames on 1-3!)
We don't have to approach even if we aren't in the lead because we can use Fireball's safely from a distance. I think Metaknight beats us more on the ground than the air to be honest. All of his attacks on the ground outrange us, and is much safer (especially f-tilt, d-tilt will put us in a really bad position) In the air a ton of aerials are quick, and have low cooldown so taking him there is better. You have to constantly play on your toe's, it's a tough match-up but use your safest options when fighting him. Also if Metaknight is d-air camping I think upB will beat this (?) Metaknight is light, stutter step f-smash kills him early, and outranges a lot of his attacks on the ground. Down-smash comes out quick as well, and it should be fresh when your playing Metaknight since most of the time you should be using aerials to rack up damage.
If Metaknight is planking then that's a different story, then I don't know what to do, as I've never experienced that so I have no clue what to do. Also I use FLUDD to push MK away if he plans to go for a edgeguard. Just make sure your high enough to use it safely, and have your double jump.
You're underestimating the tornado, and MK's approach in general. You can try to upB, but the tornado should win (upB doesn't get invincibility till the 3rd frame, I think). You could try to pivot grab the tornado, but you're gonna have a hard time just blocking it, since fireball distance starts you pretty close, and the tornado will come at you fast. Even without the tornado, fireballs are trivial for MK to get around; if we're full hopping them, we're already in the air, which is good for him, and if we're short hopping or standing with them, he can just fly on in with fair/dair/etc.
As for fighting MK on the ground...the best case for us is actually Mario in the air while MK is on the ground. Stuff like jumping nair oos. Unfortunately, he rocks us both air vs air and ground vs ground. Dsmash is hard to use because his dsmash is just as fast on startup, much faster in execution, more powerful, and much safer. We can jab into it, but it's probably best to reserve it for a kill move since he can survive fsmash surprisingly long.
Don't worry about the planking, there's absolutely nothing Mario (or most characters) can do.
Anyway, the reason I say this is hopeless is because we have no approach, have no keepaway game, and cannot even punish effectively. Nevermind the ease with which he gimps us. Sure, it's always nice if MK runs face first into an fsmash, but we have no way to win on our own power. It's totally up to MK to blow the match.
Falco's camping is a major pain yes. But luckily we have a reflector of our own and usually a fullhop Cape will stop 2 lasers from him everytime. Cape is probably your best friend here. Be extremely careful with Fireball's here as he'll reflect them, and if your camping too close you're gonna get hit.
Again, I say once Falco is in the air the advantage goes to us, we have tons of better aerials, and he can get juggled at low percents too. Fighting him on the ground is too annoying with his jabs. Offstage Falco is a sitting duck (no pun intended.) N-air goes through his sideB, if he upB's hey that's a free Cape, or FLUDD him farther from the stage and edgehog him. That's one of our major advantages here is we can edgeguard him much better than he can to us.
Be careful using the cape against Falco. If you let him get too close, he'll be able to grab you as you land. He can use this to force you to the edge of the stage, at which point, he'll have the zoning he wants. I recommend jumping and ducking instead, since you can react better if he tries to run in.
I do not agree that we beat Falco in the air. Above us, his dair is dangerous, and his bair is a solid kill move. The absolute worst thing that can happen is for us to get stuck in the air above him. We have no way to hit him from above, so all we can do is airdodge. Unfortunately, his fall speed makes it easy to stay on us until our invincibility wears off, and he lands a uair.
Falco's recovery is not weak. This is a huge misconception. A smart Falco will know when to phantasm above the stage, when to phantasm directly to the edge, and when to phantasm cancel. Yes, there will be opportunities if you can get him offstage with a good position yourself, but it's not going to happen reliably. On the other hand, Falco has a few reliable kill setups on us, including getting below us in the air, and laser-usmash.
I have no issues with Marth, close matches yes, but I don't see it that bad. Marth approaches mainly with f-air's, either run in and shield then upB, or use Fireball's. Marth has no projectiles take great advantage of this as some of his moves can cancel it out, but ya. And if he sideB's SDI and hit him with u-air works a lot of the time. And f-smash is GREAT for KO'ing, if he makes a spacing mistake he's gonna get hit with it lol. A lot of people seem to agree Marth is 65:35 or higher, I see it has 60:40, but I could be wrong .
Try playing Tiger sometime. I didn't realize it was THAT BAD until I played her. With truly perfect spacing, we can't get close, even with a perfect shield. Running in and shielding with upB is too slow; he can get behind you. Fireballs can be swatted with fair and several other moves, so they aren't a threat. As you said, fsmash is our best kill move here, and with proper spacing
and timing, we can outrange Marth's fsmash, but he shouldn't allow that to happen.
The best thing I can say we have on Marth is horrible offstage **** with the cape. Just get out there before he's in upB range of the ledge, and get in his face.
Dedede I believe is horrible match-up for us, but I've seen Mario's pull it off I wanna learn how . Someone else can help you hear for sure.
I apologize for the length of this post, and I couldn't discuss each character you listed. Also apologize if I said some things that aren't right or very clear. It's best if the rest of you help so we can get a more solid understanding . And my font color is a bit hard to read , sorry!
Honestly, I haven't played any truly nasty DDDs, it's just that we have such a hard time KOing him, and between the (infinite?) chaingrab and all around high damage/knockback moves, we simply get overpowered. Maybe someone with more DDD experience can comment.