B > B+ TheEnder suggested this to be A- but I think that's too high. You look at all the positives that TheEnder pointed out but then come to the realization that M-Beedrill has the most pitiful of defenses, an SR weakness that complicates its main usage on voltturn cores, and is prone to the majority of priority users in the tier. It can't switch into hits well barring x4 resists, it needs things like Skarmory and M-Gross out of the picture to succeed, and needs a team revolved around it to do well in most cases. TheEnder pointed out all the positive traits of M-Beedrill which would justify my suggested placement of B+ cause granted it hits wicked hard and there's a lot in the meta that doesn't want to be taking Adaptability boosted attacks by M-Beedrill. The fact that you do have to take into account all these negatives in a meta where priority is relevant and hazard stacking is a normal thing in the tier leaves me keeping this in the B/B+ range.
A > A+ This was already touched upon but I want to point out that M-Slowbro is one of the best defensive answers to things like physical variants of M-Salamence and puts a constraint on teambuilding to where you need to have a legitimate answer for this in order to not be walled. M-Slowbro walls a very significant portion of the meta and as such is a very solid choice for semi-stall and stall teams, along with balanced defensive cores. Crobro, 3 attacks + Slack Off with multiple variations of this set are all good and thrive well in the teams that these sets would occupy. Answers to M-Slowbro do not enjoy switching into Scald or any attacks coming off of a 130 sp atk with Scald having the potential to burn and cripple your answer to it. Definitely an A+ rank mon more than an A rank mon.
B- > B M-Sharpedos Crunch is extremely powerful. It 2HKOs mons such as Rotom-W and Quagsire easily and has a variety of tools to help it clean up or put dents in teams due to the nature of Strong Jaw and movepool. M-Sharpedo is a solid option for hazard stacking offense, teams with voltturn cores, and has good synergy with teammates such as but not limited to Lando-I, Raikou, Thundurus, Jirachi, Scolipede, the Latis, and so on. Speed Boost prior to mega evolution can allow it to clean up mid or late game with relative ease and mitigates the issue with Offense due to said boosts. Realistically nothing is able to switch into M-Sharpedo on a consistent basis and this includes even defensive answers such as Skarmory and Ferrothorn in the fear of running into mixed sets. Has issues with priority, still somewhat frail, and not the best of typings but definitely has enough viability to warrant its use as a B rank threat due to strength and diversity.
D > C/C+ At this point D is way too low. Dugtrio has fantastic synergy with many of the new megas such as M-Pidgeot, M-Latias, M-Salamence, M-Metagross, etc. It's ability to trap key threats to some higher ranked mons allows the player to have switch advantage on grounded threats such as Chansey, Clefable, Chesnaught, Klefki, etc. This switch advantage allows players to break down cores much more efficiently and as such has found its way on many teams that appreciate this support such as the plethora of set up sweepers in the tier. It has options to revenge kill weakened threats due to its solid speed tier and Sucker Punch, has access to Memento to make a partners sweep more efficient, and overall is a very solid option in the ORAS meta. C/C+ is where this should be.