Well, it's been over a year since this has been updated, and the meta has definitely changed in that time. With the 2U World Championship now complete, perhaps we should change it now?
I'm not a great player or anything, but I'll put some of my thoughts on potential changes below. I'm hoping it can spark some discussion.
Changes for the S-Tier
Tentacruel: S --> S+
Tentacruel isn't just a great mon in this tier, it's the iron-fisted ruler of 2U. It alone decides who is and isn't worthy to enter its kingdom. Any potentially viable mon in the tier must be able to deal with Tentacruel, speed tiers orbit around Tentacruel and being able to check Tentacruel is enough to make Pokemon more viable. Tentacruel has the tools to check pretty much every other Pokemon, and it's so good that some are willing to go into a 50/50 freeze war, just so they have a chance at beating it. Just calling Tentacruel S, isn't doing it justice.
Dugtrio: A --> S
Since Tentacruel came to the scene, Dugtrio has been becoming more and more popular as well. It's the fastest viable mon in the tier and can use its great crit-rate and decent attack to pick off and/or outright kill an opponent, making it a great revenge killer and late-game sweeper. It also has electric immunity, which Tentacruel and all other water-types greatly appreciate. Most importantly for its ranking though, Dugtrio has become a mainstay staple of the tier and is seen on every team. One must be blind to the current metagame, if they think Dugtrio should be anything less than S.
Dragonite: A --> S-
Dragonite is a powerful threat that, with some luck, can just win. It's much better than in 1U, as you have fewer checks and counters to it in 2U. I don't think anyone really needs a long explanation on why Nite is good, because it's infamous even among players who have never played RBY before. However, by the ranking's own recquirements Dragonite isn't a pokemon "that you should put on most serious teams that you create" as there are plenty of completely viable, serious teams that are better off without Nite on them.
So why put it into S-? Well, because Dragonite does "excel at [its] role in the tier to such a point that [it is a] dominating force." You can run a perfectly viable team without Dragonite, but you cannot run a viable team that doesn't go the extra mile to counter it. For example, good pokemon like Dugtrio and Persian need to run the terrible move, Toxic or else they become victim of agili-wrap. Blizzard become a serious option on Pokemon like Tentacruel and Gyarados, even if that means the mons wouldn't be running any STAB moves. Even some Kangaskhan run Blizzard, just to have the chance at sniping an opponent's Nite with a crit.
Much like Tentacruel, Dragonite is meta-warping and just calling it A is underselling that.
Changes for the A-Tier
Kadabra: A --> A+
To be honest, I'm tempted to suggest that Kadabra should either become S or S-, since it's become part of the most common core in 2U -which consists of: Tentacruel, Dugtrio, Hypno, Kadabra and a flying-type. However, I'm not sure how I feel about that. The only reason its on nearly every team is because it complement Hypno really well. Without Hypno its definietly only an A-ranked mon. Furthermore, there are teams that can go without Kadabra and still win. Does anyone else have any thoughts on this?
I don't feel like I need to justify Kadabra too much, because it's already A. We all know what it does and why its good. As said in the paragraph above, the reason it should be A+ is because it and Hypno make a great pair. Kadabra can cover your own Hypno by absorbing the Hypnosis, giving your Hypno the advantage. Kadabra also does a good job at widdling down opposing Hypno with Thunder Wave and Seismic Toss. With Hypno being such a pain to deal with, having Kadabra there is great. It's so great in fact, that I feel that the best way to contest the two is to run your own Kadabra and Hypno core, which is why we see it on so many teams nowadays.
Gyarados: B --> A
Gyarados' interesting typing has put it into an interesting position. It has become the second most viable water-type as well as the second (or possibly first) most viable flying-type. This combo, along with other changes in the tier, such as Dugtrio rising in popularity, has made it a solid contender in the tier.
Acting as an excellent check to Dugtrio, a paralysis spreader via Body Slam and a solid mixed sweeper, Gyarados has become a staple on balanced teams. This pokemon who was once a humble Magikarp can successfully claim that it has replaced the fearsome legendary bird, Articuno in the meta. So, the viability rankings should reflect that.
Haunter: B --> A
Haunter's strong showing comes from serving multiple roles at once, which puts it over the top of its competitors. Haunter's unique Ghost-typing allows it to be completely immune to Wrap and other Normal-type attacks. This means that it can do a good job at checking -and sometimes completely countering- Persian or even Dragonite. By running Hypnosis, Haunter can either serve as the team's backup sleeper or even the main sleeper if one would rather run something else on Hypno. Finally, with Thunderbolt, Haunter can fill the team's need for an Electric-type attack. Unfortunately it doesn't get Thunder Wave; however, Thunder Wave is not as spamable as it is in 1U so that is more negligible.
Although one might see Haunter as a jack of all trades and a master of none, on balanced teams where you need a lot out of one mon, this makes it better than a master of one.
Changes for the B-Tier
Articuno: A --> B
Articuno has a great special stat, decent bulk and a decent typing -with both ground immunity and STAB Blizzards. Articuno can also run Agility, mixed attacking, or Reflect/Rest sets equally, meaning it can fit a few different roles that a team might need. However, Articuno doesn't have much after that. Pokemon who are resistent to Ice-types, only need to worry about an untimely freeze. Articuno must also be careful about switching in on a potential Earthquake, because Rock Slide -which is commonly ran on those who can use Earthquake- can 2HKO Articuno without a crit. Worst of all for Articuno, Gyarados' better bulk and only 2x weakness to Rock Slide has made it a more common Flying-type choice in the meta. So, although it's not a terible mon by any means, it has become a victim of the meta changes and is no longer as viable as it once was.
Electabuzz: A --> B
To be honest, there is an arguement for keeping Electabuzz at A, but if Electabuzz stays at A then Raichu needs to be moved up to A. Because the two perform at the same level in the meta. I've gone back and forth on whether they should go up or down, but for now I'm just going to argue them both to B, because they seem to be used less in higher level play. It's not the best arguement, but I'm hoping better people can come in and share their thoughts on the matter.
Electabuzz got bumped up and Raichu got degraded after Rhydon got promoted to 1U. However, the two have proven to be at the same viability. Although Electabuzz can get some great use out of Psychic, Dugtrio -the most common Electric-type switch in- fears Surf more than Psychic. Furthermore, Electabuzz's 5 extra base speed does mean it's gaurunteed to outspeed Tentacruel; however, Raichu still speed ties and Tentacruel doesn't want to risk the 50/50. Additionally, Raichu gets Agility, making paralysis less impactful on Electabuzz.
Again, Electabuzz and Raichu perform at the same level and shouldn't be so far apart from each other in the viability rankings.
Changes for the C-Tier
Moltres: B --> C
Although Moltres was once so feared in the tier that it was banned, with Tentacruel and other bulky waters as the cornerstone of the meta, Moltres is now the one who needs to live in fear. Sporting Agility, Fire Spin and Fire Blast as its one and only useable move set it can do some damage, spreading burns against wrappers and physical attackers, trapping slower mons, etc. However, it's entirely even more luck based than Dragonite or Hydro Pump Tentacruel, and it doesn't want to spread burns to pokemon like Kadabra. It still holds a niche, but it definitely isn't at the same level of other Pokemon in the B-Tier.
Charizard: D --> C
This is the one I'm the least sure about on this list; however, I really can think of a huge difference between Moltres and Charizard. In fact, with Body Slam, Earthquake and Slash, Charizard might be a better hit and run attacker than Moltres. Pokemon who are more comfortable about switching into Moltres, like Omastar and Kadabra have to think twice. Furthermore, with 100 base speed, Fire Spin can be used as a pivoting tool more often than.
Tangela D --> C
An arguement can be made for Tangela to B as well. However, it definitely needs to be at least C, so that's what I'm petitioning for here.
Tangela can serve well on bulky teams, who need a mon that is resistent to both Ground-type and Electric-type attacks, something that Omastar desperately needs. In addition, Tangela has access to double powders, which gives a team a 75% accurate sleep move -instead of the standard 60% in 2U. Furthermore, Tangela can combine Stun Spore with Bind for hilarious psuedo Thunder Wave Drgonite-like results. Tangela's base speed is unfortunately low and even just a 8 point difference would be a huge difference on it. However, nothing can be done about that.
Overall, Tangela is a nice bulky Grass-type choice that has enough at its disposal to make it viable on bulky teams.
Changes for the D-Tier
Butterfree: Unranked --> D
Butterfree may not have anything going for it offensively, but its Ground-type immunity, double powder and a base speed of 70 all together give it a small niche. Butterfree can, at the right moment, disrupt your opponent's momentum and turn the game in your favor. Although one should have some serious reservations about using Butterfree, it shouldn't be 100% overlooked, which makes it as viable as the other D-Tier mons.
Porygon: Unranked --> D
If I had the time, will or ability to playtest 2U again, this would be one of the first pokemon I'd like to try out more, because it might just be an uncovered gem -although it'd only be like a C on this list. It holds a similar, but different role as it does in 1U. It walls physcial attackers like Kangaskhan with Recover, but it's better for spreading status with Thunder Wave, because speed matters more in 2U. Furthermore, unlike in 1U, Porygon can also kill major threats on its own. Starting with Dugtrio, Earthquake can only 2HKO with a crit, and Porygon 2HKOs with Ice Beam all the time. Porygon can also 2HKO Dragonite with Ice Beam as well. Using Double Edge, Kadabra is 2HKOed, which means a paralyzed Kadabra has to be hesitant before it switches in. Thunderbolt can also be used to 2HKO Gyarados, but that's obviously not as meaningful.
At the very least, Porygon carves out enough of niche to hold a seat in the D-Tier.
Anyways, I just wrote for way longer than I had originally planned or wanted too, so I'm stopping here. Hopefully this gets some good and proper discussion going and we can get this list updated for the current meta.