My Thoughts on the Reg M-A Meta (Early VR)
I'll start by ranking the Mega Evolutions, and explaining some of my choices. Note that the tiers aren't ordered, I just typed in whatever came to mind first. I also grouped tiers together because I haven't had the time to think the specifics all through.
| S to A+ |  |  |  |  |
|  |  | | |
| A to B+ |  |  |  |  |
|  | | | |
| B to C+ |  |  |  |  |
|  |  |  |  |
|  | | | |
| C to D |  |  |  |  |
|  |  | (No BP, the tiers aren't ordered. I think this one's much closer to C than D. Thanks for not angry reacting.) |  |
I'll explain myself for some of these.

Lopunny: I think Lopunny's definitely been underrated by a lot of players. I don't blame them, given Lucario is very powerful, but Lopunny's ability to function as an offensive utility piece is very useful. And, while it's not taking most neutral targets down, a 136 Attack Close Combat is absolutely not something you can ignore. I've found Lopunny to be the most valuable Mega Evolution in the teambuilder for me so far, though I don't think it is the "best" in the metagame overall, unfortunately. However, the hyperfast Scrappy Fake Out is simply unrivaled disruption, and the 4th moveslot has a nice variety of options like Triple Axel for Dragonite, Mega Kick if you're a lunatic, Thunder Punch for Gyarados, Encore for even more utility, and Swords Dance if you're an even bigger lunatic. For a cool team that uses Lopunny, one of them is a
sample team that I built alongside
gephicka. It's nice if you want to try out Lopunny.

Dragonite: I used to consider this to be in the S tier, perhaps a little lower, but my rating of the rain archetype as a whole has kind of fallen off. Basculegion is great, and alongside Dragonite it turns threatening mons like Volcarona into extremely easy matchups, but to be honest Swift Swim Basculegion-F is just underwhelming. With only 92 Attack, it really needs Adaptability for its Last Respects to be doing any work. And while Scarf on rain is very workable, now you're open to getting outsped by mid-speed scarfers like Krookodile, Rotom, Gardevoir, Eternal Flower Floette, or even the mirror. Additionally, while I love Politoed, it's admittedly not very strong and kind of needs a Bold nature and a Sitrus Berry to tank physical hits properly. So, nowadays I don't think Dragonite is up there in the top tier. It's still very good if you are using rain, though.

Gyarados: Gyarados is this high up not because its Mega form is that broken, but because it's a very effective user of both of its forms strategically. While you're normally seeing Charizards protecting to win a weather war, Gardevoir not Mega Evolving to keep a Traced ability, or Scovillain protecting to gain a Moody boost, Gyarados actually wants to stay in its base form a lot of the time. And its base form is great! Intimidate and its typing allows it to tank hits from a lot of high-tier threats, including Kangaskhan, Lucario, and Lopunny (unless they're using Thunder Punch or something), however it's very weak to Electric damage and also has a semi-relevant Rock weakness. Mega Evolving allows it to tank these hits better and also ignore opposing abilities. This means that, unlike most physical attackers, it's not afraid of Volcarona's Flame Body. It's also chomping right through Dragonite's Multiscale, able to OHKO with Crunch after two Dragon Dances. Clefable's Unaware and potential Water Absorb / Sturdy users are also important things you can ignore with its ability. Thus, due to having two good formes it can use, it's ranked very high.

Clefable: In a similar vein, I don't think Mega Clefable would even be worthy of C-tier if it was used purely for its Mega Evolution, but it's Clefable. Clefable is an amazing redirector in this format, so I don't think Clefablite is unreasonable if you want to increase its bulk further and actually pack a punch with Moonblast. I think Mega Clefable is really cute, so I've tried Clefablite on a few occasions, but I'll admit it's usually better with recovery items or something like a Babiri Berry.
Now, onto the general ranking. For Pokemon like Dragonite or Lucario, I won't be factoring their Mega Evolutions. However, I'll note that Dragonite probably becomes the best in the tier overall if you count its Mega form, because having 2 supreme statlines with good utility to boot is something nothing else in the tier can boast. Maybe if Volcarona could turn into a souped-up Slither Wing it could, but we don't live in that world.
I won't do as many explanations since there are very many to make, but I'll do a few.

Volcarona: In my opinion it's the best Pokemon in the format, and it's honestly oftentimes more impactful than Mega Evolutions are in certain games. Not only can the Quiver Dance set do wonderful things, but Volcarona can be used as a redirector, as speed control, it can be used as a damage mitigator, or even run recovery. The only thing that's missing is physical attacker Volcarona, but I'm sure
thebusinesspiggy will try someday. The only thing that holds Volcarona back from being totally broken is the fact Dragonite, Basculegion and Mega Glimmora are all very good. Volcarona teams can absolutely run tech to make these matchups winnable, but admittedly they make the game trickier to navigate.

Meowscarada: At first, several people said that Rotom-Mow was the best Grass-type in the tier, and I'm not going to say that it's unreasonable to think that, but in my opinion Meowscarada is absolutely better for being an absurdly fast attacker that can also serve a little bit of a utility purpose. Scarf sets outspeed every other scarfer, as well as Basculegion even when Swift Swim is active, and Knock Off is able to get rid of opposing scarves, berries, and other items. Sash sets can use Overgrow to make Flower Trick nightmarishly strong, and they can even run support options like Taunt or the pseudo-support that comes from Sucker Punch.

Zoroark: This Pokemon hasn't seen that much play so far but it's downright stupid on some teams. Scarf sets are faster than almost everything else with a Choice Scarf, and while Meowscarada is faster, it's not exactly itching to click Knock Off at a Clefable. It's also faster than the other Icy Wind / Electroweb users. Heliolisk in particular is embarrassingly 1 point slower, meaning it can't safely click Electroweb in front of the "Palafin."

Gallade: Gallade hasn't seen much play either but I think it's lowkey pretty good. It's able to set trick room and work in Tailwind at once, while most of the Trick Room setters in this format (such as Cofagrigus and Hatterene) are too slow to do so. Scarf Gallade is also pretty good, being able to outspeed and use Knock Off or Leaf Blade at Basculegion. It's a shame that the Mega doesn't have Sharpness though. Most of what goes for Gallade also goes for Gardevoir, but its Mega form is banned. It also has access to Icy Wind which is nice. That said, I've ranked Gardevoir lower because it has more direct non-mega competitors in Primarina and Eternal Floette, meanwhile the non-Mega Fighting-type pool is pretty empty.

Drampa: I don't think it's any better than B- tier, but I think people need to start considering this on some teams. It shuts down all weather with Cloud Nine, has access to great utility in Glare, Helping Hand, Icy Wind, Roost, Whirlwind, and an amazing special movepool with its STABs, and all the coverage of a classic Dragon-type. I won't list them all but you know what moves. It even gets Earth Power, which is pretty rare among specially-biased Dragon-types. A Special Attack stat of 135 is also very high for a non-mega; it's only rivaled by Volcarona, Hatterene, Chandelure and such.
Anyway, those are my thoughts on the M-A meta. I hope they age well.