I'll chime in and give what I think should be high, mid, and low B-Rankers.
Top:
Aerodactyl, Aggron, Amoonguss, Bouffalant, Braviary, Crustle, Golurk, Hitmonlee, Lanturn, Mesprit, Rhydon, Roselia, Snover, Steelix
Aerodactyl is a very good speedy Stealth Rocker, and is a pretty good lead. It also has very good speed and decent coverage to boot, which is also a plus. Aerodactyl has decent power, and its Speed is outpaced only by the mighty Accelgor, so it can deal quick damage to things. That said, it's frail, and sometimes it may not hit hard enough, therefore, high B is pretty good for it.
Aggron is, simply put, one hell of a hard hitter. His CB boosted Head Smash coming off of that spiked head is going to bring a shitload of pain to like...everything. Even if the target manages to survive the hit, they will be sure to take a lot of damage, and almost nothing can avoid a 2HKO. It can run a utility role, which is an okay set, while an RP set can definitely sweep. Its Steel typing is very good as well, and grants it some very good resistances, but also some crippling weaknesses. Still, Aggron is one of the scariest hard-hitters and Top B Rank is good; it's a force to be reckoned with.
Bouffalant is a powerful and useful Pokemon. His 110 Attack and Head Charge already make him a good hard hitter, but he also has Sap Sipper, making him a very good switch-in to threats such as Sceptile, Rotom-C, Lilligant, and Tangrowth (and Whimsicott, lol). The attack boost from it also makes him even more powerful and can smash a lot of shit with SubSD, and you'll be sure to have to sack at least two of your Pokemon in the process. The CB set is also very good and hits hard right off the bat, and smashes house. 95/95/95 bulk is nothing to laugh at either, and can take some hits, especially given that it only has one weakness. It's definitely one of the best of the B's.
Braviary is very good in RU, despite ironically being NU. It has high Attack and a very good STAB move in Brave Bird that can hit a lot of things for great damage. The Choice Band set is an absolutely incredible wallbreaker and can put sizable dents in everything, and its excellent coverage move in Superpower only comes to complement that. It can also keep up momentum with U-turn, and this allows it to scout switch-ins like Steelix and his gang. Braviary also has decent 100 / 75 / 75 bulk to boot, so he can take his fair share of hits quite well. Scarf is an okay revenge killer, and the SubBU set is decent, although not at its best like it is in NU. Overall, Braviary is a very good Pokemon and should fit in well with the Top B-Rankers.
Crustle is one of the best spikers RU has to offer. Thanks to his ability, Sturdy, and a very usable 125 Defense stat to boot, Crustle can very reliably set up numerous layers of Spikes throughout the match. Stealth Rock is a nice bonus to the cake as well, making a full hazard trap to benefit many teams. This makes Crustle a very useful lead in the tier. While he faces competition from Smeargle, he has some qualities that allow him to stand out. Firstly, Crustle sports good bulk, as well as auto-Sash in Sturdy, so he can take more hits and last a bit longer to get 'dem hazards up. The other thing that makes Crustle a good lead is that unlike Smeargle, he can pack an offensive punch, so using Rock Blast, X-Scissor, and his good 95 Attack, he can put the hurt on common leads like Smeargle (after that thing has Spored something), Snover, and Uxie. Crustle also is capable of running a deadly as fuck Shell Smash set, and while 45 Speed isn't exactly the greatest, it's good enough to allow Crustle to get his sweep going. His very good Defense also ensures priority doesn't fuck him over, bar Aqua Jet from Gatr/Tops. This thing also pairs very well with a spinblocker, and is just a very good Pokemon overall.
Golurk is a very powerful Pokemon in RU, albeit very underrated. With a titanic 124 Attack, very good STAB's in Earthquake and Shadow Punch, usable bulk, and a nice physical movepool, Golurk has quite a bit going for it. Firstly, it's a decent tank in RU, and it can check some Fighting-types somewhat well, as well as being a decent Stealth Rock user who can block Spin (but the fact that it struggles with Tops and Cryo will ensure you may need another spinblocker, such as Rotom. CB is a very powerful wallbreaker, and its power is just great. CB is an amazing set.
Hitmonlee is one powerful threat in RU who isn't to be underestimated. His high Attack, Hi Jump Kick, and decent Speed tier make him a powerful force to be reckoned with. With Reckless+LO, his Hi Jump Kick will demolish everything bar Psychic and Ghost-types, which can be solved via Absol. New Breed's Unburden set is also very cool, and using Hi Jump Kick with Unburden is nice, and also allows Lee to outspeed a very good portion of the metagame. That said, it struggles a lot with Psychic and Ghost-types, so support from the likes of Absol/Spiritomb is needed for Hitmonlee to excel.
Lanturn is a very good bulky Water-type in the tier and a usable special wall. This thing has enough bulk to take on threats such as Moltres, Omastar, and to a lesser extent Cryogonal with relative ease. It also has access to Volt Switch to keep momentum intact, while Heal Bell makes it a very good cleric. These make Lanturn a solid Pokemon. Despite this, Lanturn has no recovery, and repeated onslaughts will eventually take their toll. TO make matters worse, Lanturn won't be able to fare very well against Sceptile and Lilligant not only because he is weak to them, but because he is powerless, which his lack of any offensive presence makes him setup fodder very often.
Mesprit is extremely versatile and has a nice balance of stats. Despite competition from Uxie, Mesprit still has her place. She can run many different sets, ranging from a SR lead to a CM user to Choice. Her good 105 offenses and 80 Speed allow her to do an offensive job, doing Offensive CM very well and hitting fairly hard. Choice Specs and Scarf are also very viable and she can certainly do this well with the great coverage in her movepool. Mesprit also has some useful support options like Stealth Rock, Rain Dance/Sunny Day, and Trick Room, allowing her to be a useful team supporter who can hit back, and she even has Ice Beam to get at those Druddigons that trouble sun teams. Healing Wish is also phenomenal and allows her to be useful to get injured teammates to full condition. Mesprit also has a nice 105 Attack to use CB, and it works decently well. I think this is a high B-Rank, because despite competition, Mesprit still is very good in the RU metagame and fits in with the high B-Rankers.
Rhydon is one of the manliest Pokemon to ever be created, but he's not limited simply to being manly. Because he is an NFE, he can utilize Eviolite. With this, he's one of the most physically bulky Pokemon in the game. Using his excellent bulk and usable typing, he can check threats such as Entei and Bouffalant with some ease. Rhydon also has Stealth Rock, and is a very good user of the move and can also use Dragon Tail to phaze. Rhydon also has a high 130 Attack and phenomenal STAB's, so he can hit very hard as well. He has a number of exploitable weaknesses and is slow, plus no recovery, but his qualities definitely make him Top B.
I might get some disagreement here, but I find
Roselia to be a very useful supportive Pokemon in RU. Her access to Spikes makes her a great asset to numerous teams to lay the hazard down, which she does very reliably due to her positive qualities, of which she has plenty. Eviolite along with a Grass/Poison typing make her a good special wall, taking on some common special attackers such as Sceptile, Omastar, Galvantula, and Lilligant. Her perks don't stop there; she has access to Natural Cure, so she doesn't really care about status; and if she's forced out, status is no worry. This also allows her to effectively utilize Rest as recovery, fully healing herself while switching out to cure, as opposed to Munchlax who is dead weight as soon as he clicks Rest. She also has a good 100 Special Attack to deal good chunks of damage to the Pokemon she can take on, which is a nice plus. Roselia is definitely one of the best bulky spikers in RU, despite dropping to NU recently (Read: The only reason Rose dropped is because bad PS ladder dismisses it while using bad shit like Munchlax and Sandslash)
Snover is by himself a very mediocre Pokemon, but the fact that he commands his own playstyle is something. He is a good lead, opening up starting damage with Blizzards and Hail damage, so he can start off the match. He also allows you to throw nasty Blizzards, which Nidoqueen and Slowking can use to their advantage, as well as Escavalier working well in it. With Stallrein and many BlizzSpammers around, this only adds to the fun. Snover is also excellent death fodder, which may be needed to enter dangerous yet frail threats like Cryogonal. Snover is definitely one great Pokemon in RU.
Steelix is one of the great physical walls in RU and his menacing grin comes to the field at many physical attackers. Sporting a Steel-typing and a massive 200 Defense, Steelix can take on a myriad of physical attackers such as Escavalier, Braviary, and Aggron. His access to Stealth Rock is also helpful for the team, and shuffling with Roar is a plus. 85 Attack is kinda mediocre, but with Gyro Ball and Earthquake it does decent enough damage. Steelix is a strong physical wall, and is definitely a behemoth.
Sorry for the short post, I'll continue on with this later. But just some initial thoughts on what should be Top B imo.