I'm gonna nominate a few sets without actually detailing what they are because I'm going to assume people know the moves these sets run. In any case :
Assault Vest Azumarill --> S : Very easy S rank, great pivot, great tank, huge threat to offense, very easy to use, increadibly easy to put on a team, don't really think I need to explain this more. This is the set that makes Azumarill.
Choice Band Azumarill --> A/A+ : It's usually an inferior choice to AV since Azmarill would rather not be locked in and really loves the ability to beat Thundurus and YZard 1v1, but the extra power can be desireable, especially on sand weak team, and it's still obviously very threatening and easy to place on a team.
Belly Drum Azumarill --> A/A+ : BD is pretty hard to pull off, but it can be really devastating when you actually manage to do it. Even if you don't pull off a BD you still have Sitrus Berry which can be pretty clutch in a lot of situations. Much like the Banded set, not as good as AV, but still very good.
Every Thundurus set --> S : I really can't think of a Thundurus set that doesn't merit S. The standard TWave 3 Attacks set is a powerful attacker with invaluable utility. Nasty Plot is extremely dangerous if you can pull it off, even more so if you forgo the utility of Thunder Wave for a coverage move. Knock Off+Superpower is also great, threatening stuff such as Chansey and Latias that can usually beat you. Really, pretty much any combination of Thunderbolt/TWave/HP Ice or Flying/Knock Off/Focus Blast or Superpower/Psychic/Grass Knot is S-rank worthy, unless you pair up Nasty Plot with Knock Off or Superpower or Psychic with HP Flying or something.
Choice Specs Keldeo --> S : Literally the easiest thing in the world to use, just send it in and click Scald. Even if you don't do much damage, you'll at least have worn something down. Hydro Pump for hitting hard, Secret Sword for special walls and Water resists, then you can pick and choose between Icy Wind, HP Flying, or even HP Electric for Starmie and Gyarados. Very easy to fit on teams, very threatening and can still put in work against teams with multiple Pokemon that resist its dual STAB (ie Latias+Azumarill).
Choice Scarf Keldeo --> A+ : Pretty good revenge killer that gained a lot of popularity recently, however, it's arguably not as good as specs since it's a lot easier to switch into and doesn't really fit on as many teams as the Choice Specs set.
Choice Scarf Excadrill --> B : Not very good... Really, Excadrill needs Sand Support to work at its fullest. As a Scarfer, it's much easier to switch into and to revenge kill, and pretty much eclipsed by Scarf Landorus-T. As a spinner, it sucks becuase it loses momentum like crazy, giving a completely free turn to the opponent.
Choice Band Talonflame --> A-/A : This set kinda struggles in the current metagame. It's shifted from hyper offense more towards bulky offense, and as a result, it doesn't really cause much of a problem for most teams since most have something that can easily switch into it. Also, since setup sweepers are a little less common now, its role of revenge killer is less needed. I haven't used much of that SD or BU sets so I can't really discuss that, and the stallbreaker set is completely and utterly godly and def deserves to be A+.
Offensive LO Starmie --> B : It's just not quite as good as the defensive set. Most offensive teams can very easily fit a Lati so don't really need Starmie in the first place. It's pretty good on hazard-reliant HO which uses a suicide lead, but outside of spinning it's eclipsed by Greninja (though Analytic Hydro Pump is really strong, and Natural Cure is a great perk especially vs Thundurus, so it's not bad or anything, and defensitely has a solid niche, it's just that said nice is not quite as important as Bulky Starmie's niche of providing extremely reliable Rapid Spin support for defensive teams).
Choice Scarf Jirachi --> B-/B : This set is okay I guess, but Jirachi usually wants the bulk and recovery necessary to consistently counter MGardevoir, so the Scarf set just isn't as good as the SpD set for the most part.
Defensive Mega-Venusaur --> A- : It's not as good as the offensive set since it's far easier to switch into and doesn't fit quite as well on bulky offensive teams, but it's still pretty decent on stall and semi-stall, just somewhat eclipsed by Amoonguss.
Choice Scarf Rotom-W --> C- : Garbage set that completely forfeits Rotom-W's bulk and ability to act as a slow pivot+switch into certain threats for the ability to act as a much, much, much worse Lando-T and maybe lure Chansey. Should not be used no matter what the ladder thinks.
oh yeah and I think that we should have an E-rank for sets that are completely and utterly terrible but that some people still use for whatever reason (see: physical Greninja)