I pulled Gogoat out of a hat because there are a million Grass types and all of them are better than Meganium, but I was unaware I had to find one that was a cleric AND a phazer, so I guess you got me there, Meganium is the only Grass type that fulfills that role. Though I don't know WHY anyone would specifically need a Grass type Heal Bell/Aromatherapy + Dragon Tail user in one slot. If you don't need that, Lickilicky can run Heal Bell + Dragon Tail but also lacks Grass type's many weaknesses, is much bulkier, a little stronger, can Wish pass and can run weird shit like Explosion if you want (literally the only Magic Bounce user on the viability ranking that Meganium does better at phazing that I could find is Calm Mind Absolite Keldeo, and I don't even know if people use that). If you're only looking for Dragon Tail, Goodra and Milotic are better. I won't even name all the better clerics and Grass types.
At this point I have to ask -- is a niche that small all it takes to get ranked? Like if I told you there was a Pokémon that provided a useful Dragon immunity, resists Pinsirite E-speed, has multiple forms of switch priority, can paralyze foes through Magic Bounce without utilizing a Mold Breaker stone, and outpaces any max speed base 100 Pokémon, would you rank it? Because I just described Dedenne, another resident of PU's E rank. Pretty much everything has a "niche" except Unown and NFEs but there's no reason to acknowledge them as niches if they don't actually matter.
Let me reframe my point:
If your opinion is that Meganium is sufficiently bad that it's unlikely to be worth using no matter what unique qualities it has, then say
that. I'm not disputing that opinion, because it's an opinion, one I have no particular opinion on myself.
Do
not say that some other thing does its job better without bothering to check if Meganium has any noteworthy moves the other thing doesn't. I'm disputing your choice of comparison point because they're flat-out ignoring the notable unique traits of Meganium -Meganium can fill a niche that nothing else can beat it out at. Again, the question is whether that niche is worthwhile, not whether it exists.
Additionally, your Dedenne comparison point involves a misleading statement: resisting Pinsirite Extreme Speed is meaningless if your stats are so low you're OHKOed by it anyway.
I will say that I don't know why you'd want
Sablenite Meganium -personally, I'd assume Slowbronite would be the logical thing to go for on your Spore-immune wall-y thing that can clear Toxic etc, given that powder immunity is redundant with Magic Bounce outside of Mold Breaker- particularly given Sablenite has
so many good things to run it on that it seems a waste to put it on Meganium. Sablenite Meganium does, indeed, seem overall inferior to Lickilicky unless you want, I dunno, Leech Seed too for some reason, but Slowbronite Meganium actually seems potentially uniquely useful to me. Maybe Aggronite, though that's another hotly contested Mega Stone. (Part of why Slowbronite seems notable to me as a Meganium Mega Stone is that it's one of the more niche Mega Stones that isn't actively hampered by a bad trait or an overly specialized Ability eg Sharpedonite is unappealing if you don't have any moves Strong Jaws benefits)
Keldeo is good because its fast and can 2hko pdon, the main red orb, with Pidgeotite focus blast or Lucarionite secret sword (after rocks/any chip/modest).
It can nuke with blue orb, (which lets it at least revenge red orb), and it can make Victini miserable if it tries to switch in with an hp of your choice.
Personally I like entei, siply because it beats Zygarde. I think it is at least equally good, because not only can Zygarde not beat Entei, it also has an even worse starting type. The movepool and running adamant hurts, though- it really does.
Entei beating Zygarde is actually shaky.
252+ Atk Pixilate Entei (Altarianite) Extreme Speed vs. 0 HP / 0 Def Zygarde (Altarianite): 302-356 (84.5 - 99.7%) -- guaranteed 2HKO
+1 252+ Atk Zygarde (Altarianite) Earthquake vs. 0 HP / 4 Def Entei (Altarianite): 362-428 (97.5 - 115.3%) -- 87.5% chance to OHKO
Dragon Dance Altarianite Zygarde with Earthquake (Or Stone Edge) will beat Altarianite Entei switching in on the Dragon Dance. If it's Pinsirite Entei, Entei can't even get the 2HKO anymore and can be taken down by Stone Edge in one hit or Dragon Claw in two hits, assuming it didn't switch into Stealth Rock. In general, Entei can't really directly switch into Zygarde. (Not that Zygarde is really all that safe at switch into Entei)
In practice, Entei beats Zygarde in only some fairly narrow situations, and it has much more hatred for being forced to switch out -if Stealth Rock is up on both sides, even if we pretend Entei
does beat Zygarde outright, if Zygarde has a safe switch (Which is easy) then Entei switching into Zygarde inevitably results in Entei's death well before Zygarde.
Keldeo being able to beat Primal Groudon is noteworthy I guess, but it's also shaky -bulky Adamant Primal Groudon can survive any one hit and has a chance of a OHKO with Precipice Blades. You also are useless against Red Orb Ghost -don't laugh, Red Orb Gourgeist is a thing and actually decent- and just generally run into so many cases where you're
useless or close enough, matchup-wise.
I dunno, MnM just strikes me as one of the rare metas where Keldeo suffers far more than it gains, and while I realize it's S-rank in OU (Or was, last I heard) so
any rank other than S is a drop relative to OU, I just have difficulty imagining it in A- rank, where the rank of A is described as having "few flaws" and being able to "sweep, support, or wall significant portions of the metagame". It's got
serious flaws that apply quite consistently, and frankly everything nice about it I can think of/that's been stated by other people is pretty much directly one-upped by Absolite Manaphy. I guess it resists Stealth Rock and Manaphy doesn't?