Mega Mawile for A+
Mega Mawile is an extremely large threat with a degree of versatility and the ability to threaten every playstyle. Nothing can switch in on it, even on stall teams, and it doesn't need Swords Dance in order to 2HKO Skarmory, Quagsire, Toxapex, etc. It also can easily OHKO Dugtrio with Sucker Punch provided that its sash is broken, which is an excellent quality for breaking stall in the current metagame. It doesn't forfeit its matchup against other playstyles as SD+Sucker Punch can easily clean up games, or it can simply break teams by coming in on a choiced offensive mon (Ashninja's Dark Pulse, Tapu Lele's STAB, Pheromosa's Bug Buzz, Hoopa-U, Nihilego) and simply get a kill. Sucker Punch is also one of the most powerful priority moves in the tier, which lets it revenge kill pretty much anything when weakened enough, letting it act as extra insurance against certain setup sweepers like Volcarona, Pheromosa, or Salamence, as Mega Mawile's Sucker Punch does a little over half to each of these. It still has a few switch-ins like Heatran or Landorus-Therian, but bulky Heatran seems less common this generation, and the increased commonness of Thunder Punch makes it easier to wear it down (Thunder punch can 2HKO offensive heatran sometimes). Landorus-Therian can switch in once or twice, but it's relatively easy to wear down and take ~40% from Play Rough, or ~60% on the first switch-in if the Mawile trolls it by running Hyper Cutter. This is perhaps the best reason to run Hyper Cutter: to potentially only let Landorus-Therian switch in once.
If it has any flaws it's that it has quite low speed which makes it reliant on Sucker Punch, which can make it struggle against things on offensive teams that resist it, especially special attackers like Greninja, Pheromosa (still gets 2HKO'd lol), Keldeo and Tapus. There are less Dark-types like Weavile or Tyranitar for Mawile to check. Mawile wishes it were a bit faster too, so it can outspeed things like defensive Landorus-Therian, Tapu Fini, and other things in that general vicinity. Overall, the metagame is a little bit more unkind to it this time around. In spite of this it has a billion great qualities that allow it to be A+ rank. I actually think it's better than most of the other A+ ranks.
Mega Beedrill for.... uh.... hmm
Mega Beedrill is really interesting because in a vacuum, it's a much better mon than last generation (where it was C- rank iirc) thanks to the mega speed buff, which is tbh a huge buff and one of the main reasons it has stirred up so much discussion. But it's important to be objective when ranking this Pokemon. Disregarding the mega slot competition argument, it finds itself grossly outclassed by Pheromosa which is leaps and bounds more versatile than Beedrill can hope to be. Ultimately, Pokemon are ranked based on their niche, and Beedrill's niche has changed from "speedy momentum gaining Bug-type that outspeeds entire unboosted meta" to "Pheromosa alternative that can kill Tapu Fini better but suffers from SR weakness and is completely one-dimensional". That's not to say Beedrill is bad at its job. Whenever I used Beedrill this gen I found it performing consistently at its job, which was to outspeed nearly everything and gain momentum while dealing a lot of damage. It can't do much to threats like Landorus-T and Scizor, but that hasn't changed from last generation, where Landorus-T and Scizor were equally, if not more common and Beedrill was still seen as a niche threat that requires a lot of support but can perform well if needed. Ultimately, Pheromosa is the main thing that has changed for Beedrill. Beedrill's U-turn and Poison Jab are stronger, but Pheromosa has better Speed, the ability to surprise the opponent with its variety of sets, and better coverage overall tbh. Although Beedrill doesn't get worn down by Life Orb, this is offset by the Stealth Rock weakness. While Bug/Fighting is not spectacular neutral coverage, neither is Bug/Poison, and Pheromosa's Ice Beam + Poison Jab + Hidden Power can round it out very well. Beedrill has some decent coverage too, but it find itself competely walled by a greater number of threats than Pheromosa does, especially since Pheromosa can cheese some of its potential counters with Z-Moves. While Beedrill has greater bulk (lol) and takes roughly half what Pheromosa does from Ash-Greninja's Water Shuriken, this is not enough to build a niche on, and more importantly, Beedrill takes roughly twice what Pheromosa does from Mega Mawile's Sucker Punch, another extremely important priority attack, OHKOing it without fail.
Ultimately I can't decide whether it should be ranked, but I'm leaning towards no. If it is ranked it should be placed in C-, same as last generation. Like I said, whenever I used Beedrill it performed well at its role, but Pokemon that are completely outclassed shouldn't be ranked, even if they function well. It has a very arguable niche which I'm not sure is enough to distinguish it. It's one of the main reasons Pokemon like Raikou fell off so hard (among other things). It's just kind of a shame because it had a lot of potential this generation, and it's sure to perform well in UU (oh wait there's Scizor and discount Lando-T there too).
I had a lot of thoughts in this post and I hope I got them through in a concise way without being too much of an information overload.