So, instead of calling for Haxduck or another mon to go to S, which I personally might seem like a bit of a jump at this point (sorry Mon), I'd like to suggest that we should consider the creation of an S- ranking, to reflect a few mons that may have differentiated themselves from the glut of fantastic mons that lie in A+.
Theres one mon I've been thinking a lot about recently that probably fits this bill, which has seen a ton of increasing usage and success recently.
That mon is Aegislash
This guy is kinda nuts right now. And its not hard to see why its #6 in usage. This behemoth, boasting 3 immunities (including to being poisoned), is such a boon for an incredible defensive mon. I'd argue it might even be the best defensive mon in the game, but unlike its other defensive brethern (p2, cele, ferro), it has an ability which allows it turn to an offensive powerhouse in an instant.
Very few mons can consider themselves pure counters to Aegislash: Char-Y and Heatran, and even so, there are definitely sets that can make life hell for these two and other checks and counters. Simply put, its incredibly hard to enter a game and have a perfect idea of what set Aegi is going to be, and while you are playing rock paper scissors with some top tier mons, you straight shut down others (kartana, pheromosa, ferro, p2, Lucario, Hera). It demands a level of respect that few other mons bring to the table.
The set most deserving of S- right now (maybe even S if S- isn't created) are its toxic variants, which have been seeing explosive growth in usage, and for good reason. Being able to cripple many switch ins, along with any defensive mons that stay in, while being immune to the status itself is incredibly powerful against all forms of teams in this meta. Combine this with having the ability to deter a ton of physical attackers and a majority of choice locked mons with King's Shield, and you have perhaps the best defensive staller in the game.
However, thinking that Aegislash is a 1D mon due to its best set being defensive is to your own peril. Aegislash can run things like Ghostium-Z, Weakness Policy, or Fight-Z to decimate mons that may try to take advantage of its passiveness. And you most likely won't know until you are hit hard whether it is going physical, special, or a combination of attacking types.
Best part about Aegislash is that there is nothing stopping you from running a combination of offense and defense to whatever you desire. Want to go full offense? Use SD, Shadow Claw, Sacred Sword, Shadow Sneak. Want to go full defensive? Sub, Tox, Shadow Ball, King's Shield is for you. Want to have a combination of things? I actually have a set that is fight-Z, toxic, shadow sneak, kings shield on one of my teams, which is super good for catching opponents off guard. If that set is a little too whack for you, toxic, ghost-Z is becoming pretty standard now.
My final point will be to highlight its versatility with all of its usable moves and items.
Moves: Shadow Ball, Kings Shield, Toxic, Shadow Sneak, Sacred Sword, HP Ice, Sub, Shadow Claw, SD, Iron Head, Flash Cannon, Head Smash.
Items: Ghost-Z, Leftovers, Weakness Policy, Fight Z, Life Orb, Steel-Z.
For a "defensive" mon, thats pretty darn good. Like Greninja before it, lets give Aegi the respect it deserves and bump it up to S-/S