I'll take a jab at this.
Top A:
Abomasnow commands an entire playstyle in UU, that being hail. It makes Pokemon such as Rotom-F and Walrein much more deadly, while having residual damage that can put limits on the common Leftovers and Life Orb, which is also a plus to it. It can improve your ability to throw nasty Blizzards at the opponent, and also is taken advantage of by threats such as Nidoqueen and Slowbro. Abomasnow also has an excellent typing offensively, giving it good coverage to lay the hurt, and some useful resistances to stall with SubSeed. Definitely Top A imo.
Cofagrigus is a very useful Pokemon in UU, as it is the quintessential Ghost-type in the tier. It has a number of useful traits and fulfills many important roles in the tier. It checks the prominent Fighting-types that dominate the UU metagame, especially Heracross and Mienshao, giving it very useful utility in general. It also has special sweeping potential, having TR+NP to be extremely dangerous, and two-move coverage is certainly something that makes it no pushover. Cofagrigus is also staple on offensive Spike stacking teams as an excellent spinblocker, being able to take hits and holding its own with TR+NP and utilizing the Spike support of its own to pull off its own sweep. It's one of the most useful Pokemon in the tier and is a very top notch A threat.
Crobat is a highly unique Pokemon in the UU metagame, and fulfills a role that no other Pokemon can do. While its stats and movepool might cut into its appeal, Crobat is one to not be judged by its cover. Its Speed, along with Taunt, Roost, a decently powerful Brave Bird, and to an extent Super Fang make Crobat an anti-meta Pokemon and stallbreaker in one. Between Taunt, Roost, and its decently powerful Brave Birds, it can make stall teams cringe, as they cannot do much of anything back to it while it repeatedly wears them down. It also can check many prominent threats in the UU metagame, such as Heracross, Roserade, Mienshao, Scrafty, and Shaymin (and a fuckton more Pokemon, it would take at least 4 lines to go down the whole list), with its Speed and Brave Bird. Crobat also scouts very well with U-turn, another cool trait in its favor.
Heracross is a powerful wallbreaker, and although its Moxie Scarf set faces competition as a Scarfed Fighting-type by Mienshao (and is easily prepared for), Heracross's other sets hold much merit. Its excellent STAB combo and power make it a strong hard hitter, and Guts is a very useful ability to absorb burns from Sableye, Dusclops, and Cofagrigus while laying hard damage. Swords Dance and Choice Band are both great ways to make use of its wallbreaking prowess, and it can be a complete pain to switch into, and is great at laying the hurt. Definitely a force to be reckoned with.
Kinda like Crobat,
Mew's stats might cut into the appeal of using it, but Mew is a very good Pokemon. Its main selling point lies mainly in its versatility (kinda like Mesprit in RU), so that it can fulfill many different roles, and it is never easy to predict what exactly Mew is going to do, and even when you do find out, it's probably too late. Mew can be a boosting sweeper on both ends, and can go either physical or special, both sets being strong. It can also serve as a defensive SR user, to support its team while burning or paralyzing. It's a pretty versatile Pokemon in all and is probably Top A. Not much to say other than it's rather hard to fit on offensive teams, but either way is great.
Rhyperior is amazing in UU atm, and is just super useful. With the massive threat level of Raikou and Zapdos, as well as Rotom-H, Rhyperior does so well as a tank to take on those pesky Electric-types, being of great help for the team. Rhyperior also provides one of the best moves in the game, Stealth Rock, with only sweetens the deal and is great support. Rhyperior also has Dragon Tail to shuffle up teams. In addition, Rhyperior has good bulk and Solid Rock, meaning it can take hits very well. It also has an insane Attack and the amazing EdgeQuake STAB combo, meaning it can hit most of the tier really hard even without investment. CB and RP sets are both amazing, as the former is a wallbreaker and has no true counters, while the latter is a pretty strong sweeper overall. In addition, much like its pre-evolution, Rhyperior is extremely manly, and this gives Rhyperior an edge over most of the A-Rank Pokemon, and is another reason to put it in Top A.
Roserade is the former #1 Pokemon in UU, and is worthy of that former glory. It can serve as both an offensive powerhouse and useful team supporter, and as such, it can very often be what you're looking for. Roserade has awesome power in Leaf Storm and Sludge Bomb, as well as Extrasensory/Hidden Power to work with, and with either Choice Specs or Life Orb, Roserade makes an amazing wallbreaker. It also has Sleep Powder on the wallbreaking sets to essentially eliminate a counter, which only sweetens the deal. It also has Natural Cure, and it can run either Synthesis or Rest+Natural Cure to work with, and keep itself healthy. Natural Cure also means Roserade is a great way to absorb status, and help for teams that dislike Scalds and Sableye/Dusclops. It can also support its team with its usable special bulk and typing, while laying down Spikes, a useful entry hazard to have. It also has Aromatherapy and Toxic Spikes to boot, which helps. (oh and Leech Seed lol). Overall a very useful Pokemon and is definitely among the top of A-Rankers imo.
Sharpedo is the epitome of late-game cleaning, even moreso than Krookodile and Honchkrow. Its ability, Speed Boost, is one of the best in the entire game, and once Sharpedo gets one or two boosts, it's usually game over. Its useful offensive typing and high Attack mean it can handily sweep down many teams, and with hazard support alone it's a really neat win condition. It also has a good Special Attack, so it can run a special set as well, that has the ability to break down physical walls as well as the surprise factor. One of the best late game cleaners and possibly broken, a Top A-Rank mon imo.
Mid A:
Azelf is probably a Mid A-Rank Pokemon imo. It's a really good Pokemon on offensive teams, and has a lot going for it. It can be a really dangerous sweeper with NP, and its high Speed and power mean it can be really hard to stop, and its coverage is nothing to laugh at either. It also can shine as a Stealth Rock lead, and can Taunt+SR to be a suicide lead, or just use SR+3 Attacks to support the team but also lay some hard damage. It also has a viable CB set to revenge kill, hit hard, and have surprise factor, as well as Dual Screens and weather to support its team decently.
Flygon has a rare Ground / Dragon typing, giving it very good coverage in UU and make it a great hard hitter. CB Flygon is great for EQ+Outrage, and breaks down teams like a complete boss. EQ also is great because it can pummel incoming Cobalion, Bisharp, and Empoleon. It's also got U-turn to switch in and out of battle to scout, and Levitate+SR resist means it doesn't give two fucks about entry hazards. It's also decently bulky, and can take a couple of hits, and the only thing that can OHKO it at full HP is probably Reckless HJK from Mienshao. Scarf Flygon is possible for revenge killing but is subpar otherwise.
Slowbro is one of the better physical walls in UU, and can handle common threats such as Mienshao and friends pretty well. It has a lot of bulk and Regenerator+Slack Off is great so it can constantly stay healthy. It is also a great pivot with Specs, as it can come in, hit something pretty hard, and switch to constantly stay healthy. Its weaknesses are unfortunately very exploitable, with threats such as Heracross, Chandelure, and to an extent Sharpedo/Krookodile.
Ughh,
Umbreon is a very annoying Pokemon in general. It is extremely bulky on both ends, especially the special side, and is a bitch to take down by most special attackers, bar Chandelure. It is the single best special wall in UU, as it can take many hits for weeks, and it can take a couple of physical hits too by virtue of its great Defense. It supports its team so well with Wish and Heal Bell, and keeps your team pretty healthy; the latter along with Synchronize make it hard to take down by status. Its access to Foul Play is also amazing, so it can heavily dent incoming physical attackers.
Weavile is a great physical sweeper, having one of the best Speeds in the entire game. While its typing is atrocious defensively, that has almost no impact aside from the SR weakness, because Weavile is frail anyways. It has all the tools it needs to be a strong offensive threat: Speed, high Power, Swords Dance, and great coverage means Weavile can handily sweep a number of teams. Its STAB Combo is also great in that none of the physical attackers resist the combo, and as such they take heavy damage from Weavile. Weavile is also one of the better Pursuit trappers in UU, as its Speed, power, and with a CB intact it can trap common threats such as Azelf, Mew, etc.
Low A:
In terms of pure power,
Darmanitan is right up there. Its 140 Attack, and SF+LO Flare Blitz is insane, and Darmanitan can flat out destroy most of the tier. It has Superpower, QuakeSlide, and U-turn as well for coverage options. It is also immune to burns, and has an amazing amount of power to wallbreak in general. Its Speed tier is not too bad either, so it can outspeed a number of things, especially Chandelure and Heracross. The main thing putting this at Low A though is its issue with being worn down really easily.
Shaymin is a pretty good Pokemon, with a great Seed Flare to work with as well as useful coverage options in Earth Power, Psychic, Air Slash, and Hidden Power. It can hit pretty hard, and has Synthesis and Rest+Natural Cure as recovery. It also can run SubSeed to be incredibly annoying, and has pretty good bulk overall. It also has a fine Speed tier to outspeed a couple of things, although Mienshao and the less common Scolipede can be problematic. Shaymin also has Natural Cure to absorb burns for those kind of teams, which helps. Shaymin faces very stiff competition from Roserade, however, who has much more power, Spikes, Sludge Bomb, and Sleep Powder. Nonetheless, Shaymin is still really good, and fits fine in Low A.
Slowking is a really good Pokemon on balanced teams. It has a useful defensive typing allowing it to take on prominent Fire and Fighting types that take prominence in UU, and has good bulk overall as well as great utility in T-Wave, Dragon Tail, and Scald. It also has Slack Off+Regenerator. Slowking has better special bulk than Slowbro, which make it a good mixed wall, while makes it by far the best matchup against Kingdra; if you're team really wants something to handle Kingdra better, you can count on Slowking. It can take a hit from it and retaliate with Dragon Tail/Thunder Wave, and even if it will sometimes lose one-on-one, Slowking helps greatly so it will leave Kingdra weakened to the point where your team can handle it quite easily. Dragon Tail is also useful to make it harder to switch into than Slowbro, and it can rack up hazard damage. It can also run TR+NP and be dangerous, just like Cofagrigus. It has some competition from Slowbro, but Slowking excels in utility where it lacks in pure bulk.
Swampert is a great check to Electric-types, as well as setting up Stealth Rock. It has a useful STAB combo and decent power to boot, and has a rage inducing Scald. It also has the ability to take a lot of hits, and can phaze with Roar. It can also run CB, and it's a great hard hitter. It does kinda struggle in a Roserade+Shaymin prominence meta, although it's still really good.
Venomoth is annoying as fuck, because it can incapacitate something with Sleep Powder while also Quiver Dancing, gaining power and Speed; and it can pass those boosts to strong special attackers to make them complete ownage, and allow them to become hard to stop. Its use is limited to full on special offense teams though, and it's pretty one dimensional, but nonetheless a bastard Pokemon.
Just my two cents.