I'd like to address the issue of whether Megagross should drop to A+ , as i noticed Megagross's pros and cons were slightly exaggerated on both sides in some of the posts. With that said, I will declare that Megagross should remain S rank.
Now Megagross by no means is sweeping the tier as flawlessly as it arguably was a few months ago when it was called to a suspect test. As indicated in the sets viability rankings, megagross's AoA attacking wall breaking set is arguably its best set, where its dual stabs 2hko and even ohko a lot of mons in the meta, and then it has some nice coverage options in thunder punch , grass knot, ice punch , hammer arm, and EQ, allowing it to break balance cores effectively and get by its common checks, and is one of the best pursuit trappers in the tier. Lesser used A rank sets such as rock polish can be very effective, as not much can stop + 2 speed base 110 speed mon with amazing bulk , typing, and tough claws backed up by base 145 attack. However even with these options, Megagross recently has found itself some road blocks. Most regular defensive slowbros are now running evs in spdef to avoid the 2hko from grass knot, and thanks to regenerator can be a good pivot into Megagross, and can threaten it back with fire coverage and or scald burns, even thunder wave. Calm mind slowbro variants including the mega will be able to also be able to put up a good fight against Megagross. Similarly , AV slowking outright walls Megagross as well if it lacks thunder punch. In addition to this, skarmory generally walls Megagross lacking thunder punch, as it will not lose to hammer arm since most skarmory are running counter. Mega scizor who has spiked in usage and is one of the better A + mons atm , and essentially uses Megagross as setup fodder for its bulky SD sets. Mega Gyarados , both pre mega and post mega is a decent check to Megagross with intimidate and also walls its dual stab and threatens it back with crunch, but of course doesnt want to take a hammer arm or thunder punch. Bulky Chomp with rocky helmet is as popular as ever and really punishes megagross and wears it down quickly if it lacks ice punch. Several Victini variants are a good check to Megagross lacking EQ, and threatens to ohko with v create. Jirachi also can wall megagross lacking EQ similiarly, and the nearly omnipresent Sand rush excadrill is a pretty easy way to put an end to a Megagross's momentum, on a safe switch of course.
But even with these flaws, Megagross is still an elite mon in this tier, can get by most of its common answers, and is certainly better than most of the A+ mons. It also still has certain metagame trends that are benefiting it, such as the spike in usage of mega venusuar which is arguably the best answer to mega altaria in the tier. Megagross is still among the most reliable checks to its S rank brethren, Clefable and Mega Altaria. Its positives do in fact outweigh its negatives, So keep megagross S.