The goal is to make every character equal (with a few notes about transforming characters who are in a kinda odd place). That's impossible, but it doesn't mean I'm not going to try to get as close as I can. Why would I, in my right mind, intentionally make Meta Knight better than the other characters? I'm not going to intentionally make him bad or anything either, but no character is entitled to top tier.
MK's matchups haven't been thoroughly explored, but here are the ones I'd look out for:
Bowser/Donkey Kong: They're listed together because they have the same concepts here. Both really didn't do that badly against Meta Knight in the first place, and the changes to Mach Tornado make their sizes way less of a liability. The power and weight difference between them and Meta Knight is just so extreme too, especially now with them getting better and Meta Knight's kill moves getting much worse; Meta Knight is really going to have to either set up and land the gimps consistently or play a very good game of "don't get hit" here.
Yoshi: This was always a weird matchup, but it gets weirder. On one hand, Yoshi lost his cg which was really useful against MK. On the other hand, Yoshi's "I don't die" factor combined with Meta Knight's newfound "I don't kill" factor is going to give Yoshi a seriously significant advantage in attrition-based gameplay (remembering that Yoshi has a great projectile while Meta Knight has none is important too); I bet a Yoshi who positions smart, camps hard, and has exceptional DI and momentum canceling would be really hard for MK.
Ganondorf: Did I really say that? Yes I did. The main issue Ganon has to look out for is being gimped; MK tends to have an easy time of that. However, if he can control center-stage (or if he cps a walk-off stage or something like PictoChat even with small off-stage areas), suddenly the power is on his side as his hits really mean something in a way MK's really don't. The main nice thing for Ganondorf is that, assuming we factor out ledgestalling here, Meta Knight's ability to play keep-away against him is not on par with most of the cast. In the air, Meta Knight is about as mobile as Ganondorf, and if Meta Knight wants to attack, he has to get decently close to Ganondorf which is always a much scarier place to be than, say, half the stage away.
Those fun cases aside, basically everyone sees a better MK matchup so just generally investigating the old high tiers will probably find you qutie a few positives. A2 brings up the MK vs Olimar matchup and how MK's changes really help Olimar. I can't say I've explored that one in depth, but what he's saying makes sense and in light of how Olimar already did decently against MK a + for Olimar now wouldn't really be surprising. Even random characters like Zero Suit Samus have just a strictly better time; she has basically an extra move (fsmash), paralyzer -> grab, and a better dtilt while he has to work harder to kill her. The ZSS people already thought they had a close matchup with MK so what do you figure that means? I just picked ZSS at random too; she's not notable in gaining anti-MK abilities but just makes you think about that.
Though speaking of Zero Suit Samus, while looking up what her board thinks about her MK matchup, I happened upon a suggestion that DK has a 0-70% chaingrab on her with dthrow. Really now? Does anyone know about this? It's news to me, but it's good I found out about it now if it's legitimate. It's bizarre that it would only work on Zero Suit Samus though; she doesn't have noteworthy physical properties that should make her a unique victim of chaingrabs.
Also steeler, HDR did in fact nerf the top tiers. The top tier in ST is Balrog, Dhalsim, Vega, and O.Sagat. In HDR, O.Sagat doesn't exist anymore and Sagat can only be played as an improved version of N.Sagat (the "old" characters in general are gone, though this only really matters for Sagat, Ken, and THawk). Balrog and Dhalsim both had powerful throw loops removed and had their supers get damage reduced from 60% life to 50% life (Dhalsim also got the amount of invincibility on his super slightly reduced). Dhalsim benefited from easier inputs, but in what is doubtless an unfair exchange Yoga Teleport got directly nerfed with more vulnerable frames. Vega's wall dive no longer knocks down which pretty much totally changes his game in a way that's not beneficial to Vega. He also lost his ability to store a charge on his flip kick which isn't a big deal but is a nerf. I guess they weren't severely nerfed, but to say they weren't nerfed would be inaccurate. I've also seen really disparate opinions on the balance of HDR. Some tier lists for it look kinda like ST tier lists except more compressed, and others aren't even close to similar with stuff like Bison in top. The only things people really seem to agree on for HDR's tier list are that Akuma is still too good and that Cammy and Fei Long are still near or at the bottom (though about a million times more viable than before).