Nominating Mega Medicham to A+ rank I'd say S-rank but I'm not too sure about that yet but yeah not much can wall this and it's the reason things like Mew, Cresselia and Slowbro are seeing more usage. Notice how they are all weak to Pursuit so it can easily be paired with a Pursuit trapper like Bisharp or Tyranitar so only Mew and Sableye are good at taking it on. Offensive teams and stall both struggle with Mega Medicham and it causes a lot of switches giving it free opportunities to set up a sub. Like for example you're facing an offensive team and since nothing on their team can wall it they have to play around with switches and they switch to Dragonite on the predicted High Jump Kick then hopefully finish it off with E-Speed you can just use Substitute to kill it with Ice Punch and avoid E-Speed damage.
Nominating Gliscor for A+, SpDef Gliscor is amazing I use 252/252 SpD for maximum bulk and it can even stay in on standard Rotom-W, hit it with a Toxic and stall out it's Hydro Pumps with Roost. It can also stall out Slowbro's Ice Beam, stay in on LO Latios Draco Meteor and stall it out with Roost or Toxic it, it also walls threats like Tyranitar even Ice Beam variants, Alomomola, Quagsire, Chansey. It can also stall out Suicune with Taunt, Toxic and Roost. Other things it walls are the extremely common Landorus-T, Heatran, Terrakion, Mandibuzz, Landorus-I and more. Did I mention it's also an amazing stall breaker? It doesn't give a shit about status or Knock Off if Toxic Orb has already been activated. Taunt, Toxic + Roost is a nightmare for stall to face.
Nominating Gliscor for A+, SpDef Gliscor is amazing I use 252/252 SpD for maximum bulk and it can even stay in on standard Rotom-W, hit it with a Toxic and stall out it's Hydro Pumps with Roost. It can also stall out Slowbro's Ice Beam, stay in on LO Latios Draco Meteor and stall it out with Roost or Toxic it, it also walls threats like Tyranitar even Ice Beam variants, Alomomola, Quagsire, Chansey. It can also stall out Suicune with Taunt, Toxic and Roost. Other things it walls are the extremely common Landorus-T, Heatran, Terrakion, Mandibuzz, Landorus-I and more. Did I mention it's also an amazing stall breaker? It doesn't give a shit about status or Knock Off if Toxic Orb has already been activated. Taunt, Toxic + Roost is a nightmare for stall to face.