Here are my opinions on the current S tier Pokemon.
TOP
Nidoqueen and Slowking
I dont think anyone will disagree with Nidoqueen being in top S rank, its easily one of the best Pokemon in the tier and probably the most controversial Pokemon in the history of RU overall (just look at the suspect discussion x_x). Her excellent type coverage combined with her ability in Sheer Force make her an excellent wallbreaker, which is further augmented by her resistance to passive damage such as Toxic, Stealth Rock, and Thunder Wave, as well as her useful resistances to Electric and Bug (she has quite a few other resistances too, but most of the other Pokemon that carry those moves usually run a way to deter Nidoqueen anyway, making it sort of a moot point.
Slowking is the very definition of "metagame defining", in my opinion. Slowking's been around for a very long time, and except for very early on when Yanmega and co. ran rampant, its been one of the, if not the most influential Pokemon in the tier. Slowking is capable of running a ton of sets such as its standard defensive set, Choice Specs, Offensive Trick Room, and even Calm Mind, making him a great option for offensive and defensive teams alike. Slowking has excellent defensive stats all around and a good support movepool, which when combined with its cool typing and one of the best abilities in the game: Regenerator makes him the single best pivot in the entire tier, and a major roadblock for multiple threats such as Entei, Moltres (needs to be slightly more careful due to hurricane), Hitmonlee, Emboar, and Cryogonal. Although the metagame isnt as kind to it as it was in BW1, i feel that Slowking's countless positives outweigh that enough for it to be placed in top S rank.
MID
Uxie, Entei, Sceptile, Druddigon, and Escavalier
Uxie is gifted with what might possibly be the most expansive support movepool in the entire metagame, including moves such as Stealth Rock, U-turn, Dual Screens, Toxic, Thunder Wave, Memento, Heal Bell, Trick Room, Rain Dance, and Sunny Day, as well as some of the highest defensive stats in the game, sitting at 75/130/130. These two traits make Uxie hands down the best pure support Pokemon in the tier, almost always being able to do its job (sometimes multiple times per match!), and setting up win conditions for its teammates quite easily. Not to mention Uxie's good speed and excellent defensive stats also make it the perfect candidate for a deadly SubCM set, which is one of the most underrated yet effective sets in the current metagame in my opinion.
Entei is, simply put, an absolute monster, and one of the main reasons why almost every successful team needs to run a Fire resistance. The power of Entei's Flare Blitz is truely a sight to behold, cleanly 2HKOing almost every Pokemon that doesnt resist it, while Extremespeed makes Entei an excellent revenge killer, capable of picking off a slew of threatening Pokemon before they can do too much damage such as Accelgor, Galvantula, Sceptile, Absol, and Lilligant. As Cherub Agent mentioned before, the main and possibly only reason its not top S rank is because of how quickly its worn down from a combination of Flare Blitz recoil and hazards damage, making its lifespan quite short if the Entei user doesnt play well (also i guess a lot of common Pokemon such as Slowking, Qwilfish, Druddigon and Poliwrath wall it, which further hinders Entei from being top S rank material.
Aside from being literally 2hko'd by Stoutland's Fire Fang, Escavalier is quite the juggernaut in the RU tier. Nothing wants to take a Megahorn off of that huge base 135 Attack Stat, and while speed isnt its strong suit, Escavalier's natural bulk and good defensive typing more than make up for the speed issue, granting it free switch ins on a ton of common Pokemon such as Uxie, Hp Rock Lilligant and Sceptile, Choice locked Druddigon, and Cryogonal. Escavalier also happens to be one of the best hail checks in the tier, being immune to the residual damage caused by Hail thanks to Overcoat, while using its natural bulk and resistance to Blizzard to neutralize Hail abusers such as Glaceon and Walrein with ease. Escavalier's low Speed stat still gets in the way occasionally, as well as its somewhat limited type coverage, but its positives certainly qualify it for mid S rank imo.
Sceptile is one of the best sweepers RU has to offer, and its easy to see why, Sceptile can run both physical and Special sets effectively, having close to 0 counters between the Special Attacker and Swords Dance sets. Sceptile is also one of the fastest Pokemon in the game, outrunning almost the entire unboosted metagame without a boost, making it that much harder for the opponent to deal with it. The main thing keeping Sceptile out of top S rank imo is its fraility, especially on the physical side, 70/65/85 isnt winning any awards defensively, and Sceptile often has trouble switching in against more offensive teams in fear of simply being OHKO'd on the switch in without accomplishing anything. Sceptile's fraility also makes it more vulnerable to priority such as Entei's Extremespeed, which can stop a Sceptile sweep prematurely quite easily.
As for Druddigon, its a bulky Dragon-type with insane damage potential, a great offensive movepool and access to Stealth Rock, arguably the best move in the game in a metagame where FWG cores are incredibly common, what did you expect? I could say more, but its like 4 in the morning and i think its pretty self explanatory tbh :x.
LOW
Moltres
As Cherub Agent, Nails, and Swamp Rocket mentioned in their posts, while Moltres is easily one of the most dangerous Pokemon in the tier when left unchecked due to its raw power and great dual STAB, that Stealth Rock weakness is incredibly crippling, and prevents Moltres from being top S or even mid S imo. Stealth Rock is on almost every team, and while its relatively easy to keep Stealth Rock off the field in RU simply because of how good Kabutops and Cryogonal are at spinning, it still needs a bit more support than the other S rank Pokemon to function to its full potential, and thats what prevents it from being any higher than low S rank (tbh if Stealth Rock didnt exist Moltres would probably be bannable, lol).