Gengar dropping is out of the discussion imo because of how versatile it is. Sub Wisp + Hex, LO 3 Attacks + Taunt/DBond/Sub, the ancient Sub Disable, Trick Scarf/Specs, or even the weird ass Lead Sash + Counter set. You really can't build and say you have something to reliably switch into Gengar because nothing really does unless you directly know the set. Mandibuzz w/ SpDef, Heatran, Tyranitar, and Ferrothorn are some of the best mons to check it, but Ferro loses to Sub Disable, Tyranitar loses to Wisp or a Focus Blast on the switch-in, Mandibuzz loses to D-Bond if it Foul Plays, and Heatran dies to a LO Shadow Ball followed up by Focus Blast. Gengar is just a really annoying and hard mon to play around, especially because of the Ghost buff. I think it's good at A+, especially since you usually can't even predict the set from team preview depending on your opponent's build, and it can either break offense or stall.
Landorus-I should stay A+ because of how good its Calm Mind set, and even it's Knock Off +3 Attacks set is right now.
It really just eats common Stall builds alive rn, not to mention Balance. Tentacruel, Chansey, Jirachi, Quagsire, Skarm, Mandibuzz, Slowbro, M-Altaria, M-Sableye, etc. all are just a light snack to Landorus. Unless you have Celebi (assuming no Sludge Wave or Knock Off) or Cress (assuming not CM + Sludge Wave, which won't matter if it has Ice Beam) on your stall, Landorus really can break through you with a bit of support. It's also pretty good against offense since the combination of Earth Power and Psychic destroys a lot of common mons without much prediction being needed. I think Landorus is as good as it was around mid-XY rn, because the most people do to prepare for it is slap a Lati on their team. It's speed tier is lacking but it's great a great mon atm.
I think M-Pinsir should drop to A. While Pinsir does shit on new megas like Lopunny, Sceptile, Beedrill, and Gallade, the ubiquotous Landorus-Therian amongst others can beat it very handily, although they nothing barring a healthy Zapdos or Rotom-W wants to switch in right away. The thing about Pinsir is that it wants to come in once it can SD, and before it mega evolves, and either sweep from there or break everything to the point that a cleaner can finish up the remnants of teams because of its crippling weakness to SR that prevents it from switching in and out as it pleases. But Scarf Tyranitar, Scarf Magnezone, Slowbro, Rotom-W and Landorus-Therian are so common that it's hard for Pinsir to break apart teams like it could during XY. Plus rain is fairly common, which means Kabutops/M-Swampert can eat up +2 Quick Attack reliably and proceed to waterlog its oblong face in. And Talonflame is more common now because of how good it is atm, and Pinsir can never beat it in a 1v1 circumstance. It's still a good mon, but i feel like people check it without even taking it into consideration when teambuilding.
I'm a bit mixed on M-Slowbro. It's better than some of the A+ mons but at the same time just as good as a lot of the A mons.
Its Scalds hit surprisingly hard because of it's SpA boost, and its bulk allows it to eat up any physical hit that isn't STAB, boosted or super effective with complete ease (i actually remember thinking i tossed a stall team pretty easily with my M-Gross until i realized that EQ doesn't even do the 20% i needed to knock out M-Slowbro, which cost me the entire game. and so i curled up in a ball and cried a bit). It beats practically all CM users, barring M-Sableye, M-Latias, and Unaware Clef, because of Shell Armor, and has a ridiculously easy time setting up. It even has utility early to mid-game before mega evolving because it still checks a lot of mons in the tier and has Regenerator to gets its health back up so it can CM later, although it not having T-Wave means it can't fully check some mons like Zard X. But there's not much reason to use it over M-Sableye on stall since M-Sableye beats a majority of hazard setters (Spikes + well played Offense = easy win vs stall if matchup is decent), as well as spin blocks for hazard stacking stall, and just supports stall arguably better than M-Venu did early to mid-XY. Plus M-Sableye's bulk also helps it to set up on a shit load of mons if it's CM, and it has the utility of burning anything it wants and priority recovery before it mega evolves and the turn of. Regular Slowbro kinda outclasses M-Slowbro as just a physical wall since it can switch out and get back 33%, whereas M-Slowbro has to directly go for Slack Off to get back HP to check things later on, meaning you have to recover and can't just double if you predict a switch to keep pressuring your opponent. It has its uses but i don't think stall has much of a reason to use it over M-Sableye, and regular Slowbro is kinda more useful on balance since it has T-Wave (assuming you're Calm Mind) and Regenerator. It's still a viable win condition nonetheless but i could see it going either way, though i'm kinda leaning towards dropping it.