B+ Tier
MinunA slightly less strong, slightly bulkier version of Plusle. Works as a superior agility pass lead on Baton Pass, works as a superior screens setter. Is an inferior Petaya Berry abuser. Overall it is seen a lot less often, and on more niche builds. But I can't justify putting Minun a lot lower than Plusle when they are so similar.
CrawdauntA simple Surf, Crunch, Ice Beam, Knock Off set can be absurdly effective. Comes into Chimecho for free, and comes into Metang easily too (and with some speed investment you outrun max speed adamant Metang, makes the MU safer). Breaks down defensive cores like nothing else. Has no switchins because of Knock Off. Coverage Ice Beam destroys Roselia. More sturdy grasses like Bellossom are permanently hindered by Knock Off. Just a very strong wall-breaker that fits the meta game well, courtesy of Chimecho and Metang.
DewgongCompetes with Wailord. STAB Ice Beam is a great differentiating factor. My favorite set is Surf, Ice Beam, Rest, Sleep Talk with max HP and max Spa Modest. Problem is, Dewgong can't fit all the moves it wants. Without HP electric, you can hardly say Dewgong checks opposing waters. Dropping Rest Talk hurts, because on an offensive water with three attacks, I'm usually inclined to swap them out for Wailord and put Self-Destruct in that last slot. Still majorly threatening, good on balance. Perish Song is cool utility but I find it hard to fit. Encore is good too, but I find it hard to justify when the only real target is Calm Mind Chimecho, who can easily Psychic on the switch as Dewgong comes in. Typing has some shortcomings. Meteor Mash neutrality makes them unreliable as a sole CB Metang check. Fighting weakness means you need to pair them with a Chimecho or Tropius (or another Hitmonchan check). This isn't hard to do, considering Chimecho is a top mon.
OctilleryNothing in the tier wants to switch into Octillery. Surf, Fire Blast, HP Electric cover all the relevant bases. Thunder Wave nails anything that isn't immediately threatened by Octillery's coverage. In fact, he feels like the most reliable paralysis spreader in the tier. He is also the best baton pass recipient in the tier. He doesn't see much usage because of competition with Wailord on standard teams. On teams with slow wall breakers, I can see Octillery facilitating their effectiveness in a way that Wailord can't match. He is an excellent lead as well, I'd say better than Wailord in that role.
GolbatSurprisingly great defensive profile, reminds me a lot of Haunter. Spikes immune offensive menace. In the context of the current meta game where Mawile usage and Relicanth usage are low, his lack of Earthquake is much less of a hindrance than it would be otherwise. Hidden Power Ground is sufficient for breaking Metang, easily. Has trouble breaking Torkoal.
VigorothHard to justify putting Pidgeot and Raticate this high and not Vigoroth. Access to Bulk Up, Earthquake, and Slack Off are all huge for a physical attacker in this tier. He packs longevity and power. Finds himself being free entry for Haunter often, because it is so hard to justify dropping his excellent moves to make room for Shadow Ball. People do run Shadow Ball sets, but I personally struggle with it. Doesn't have great points of entry, requires some aggressive play to get onto the field. Harder to put on a team than Pidgeot. The Spdef set has better points of entry, but the loss of speed hurts. Lead set can deny Glalie spikes and hurt all of the other leads reasonably well, but generally another lead is better.
RaticatePidgeot is largely superior as a CBer in my opinion. The Reversal set is what I'm impressed by. It is incredibly threatening, in part because the opponent may expect Choice Band and allow Raticate a free turn to lay up a Substitute. The Reversal set also destroys traditional CB Raticate hard counters like Relicanth and Mawile. Better of the two sets by far.
MurkrowVery dangerous offensive threat. Drill Peck can two hit KO anything that isn't a flying resist. Checks both Chimecho and sweeper Bellossom. My main problem with Murkrow is that one poor prediction means death. That is a dangerous prospect when it is their job to check such imposing threats. Haunter has a similar problem, where one poor prediction means death. However, Haunter's defensive profile is a lot more dynamic (immune to normal, ground, fighting, quad resistant to poison), and Haunter has freedom to choose any item it wants, and keep the opponent guessing if he'll attack or substitute or status. Murkrow needs to run Choice Band.
CacturneGood mon, checks Chimecho, Haunter. Mixed sets with Focus Punch are fun, as are full physical sets and Sand Veil abuse sets. Spdef set can check Wailord to an extent, but with no synthesis he can't last forever; needs secondary support in that role. Harder to throw on teams than Roselia.
B Tier
ArbokIn a meta game where Mawile and Relicanth are more common, I think Arbok would be superior to Golbat. However, with the most common normal resist being Metang, Golbat has an edge right now. Arbok is spikes vulnerable and weak to Hitmonchan's common Earthquake coverage (Golbat is weak to Rock Slide from chan but that is much rarer). He is slower than Golbat (and slightly stronger), but realistically that just means he misses out on the base 90 speed ties. Nothing with base speed between 80 and 90 is too relevant. Leftovers Glare sets have potential.
MagcargoBest Flareon counter in the entire tier. Good glue mon, combines the defensive profiles of Metang and Torkoal. Needs some team support to cover fighting, ground, water weaknesses. Pretty underrated. With the rise of Flareon to S tier in a lot of peoples' minds, and the influx of Raticate, Pidgeot, Murkrow I believe Magcargo is a heads-up option in the current meta.
WhiscashReally good Metang check. Also checks Haunter fairly well (unless it is Giga Drain). Only itself, defensive Tangela, and defensive Chimecho can be called legitimate Dragon Dance Pupitar checks. Issue is that it can compete with Wailord for a team slot and gives Bellossom, Cacturne, Chimecho easy entry. Earthquake nails Roselia, and EVing to outspeed Rose is feasible, so at least that isn't a problem. Just more specialized and harder to build with than Wailord, and has a smaller role in the scheme of the meta game because of that.
PelipperSingle best Metang check in the tier. Also the water-equivalent to Tropius, in the respect that it is a specialized Hitmonchan check. A water type that can't check waters (because HP electric coverage). Good mon, but requires team support. Also can potentially run an agility set and sweep (but Huntail is better for that role).
PiloswineUnderrated. The destructive power of Choice Band cannot be overstated. Piloswine finds its way onto the field vs. Haunter, Plusle/Minun, and resisted poison attacks. Always leaves a sizable mark on the opponent. Can also run a cool Light Screen lure set.
DiglettI'm not a huge fan, but certain teams facilitate him. Diglett is only good with a select few partners. Notably, BP Mawile, BP Flareon, BP Plusle, and Encore users. On teams without these mons, Diglett is strictly on revenge-kill duty. Since mons like Chimecho and Haunter can easily take advantage of a Diglett locked into Earthquake, I'd say choosing a better revenge killer that doesn't enable these threats is preferable.
KinglerUnderrated offensive threat. Also runs a mean Knock Off set on stall
KecleonChecks Chimecho and Haunter more reliably than almost anything. Good pokemon but I find myself being taken advantage of when I run it. With Flareon commonly running mixed sets, he can't reliably handle them. Unless you are Focus Punch, Pupitar gets free setup in front of you. Torkoal and Metang can come into Kecleon repeatedly with little fear (unless the Metang is offensive and dislikes being paralyzed). Checks sweeper Bellossom, and is an effective secondary check to the offensive waters.
RelicanthHas some excellent traits, but his typing means he can only really check normal/flying attackers and Metang. Fires destroy him with HP Grass coverage. Relicanth usage is on a down swing currently. If the normal attackers really pick up steam, I can see him climbing back. EVing Relicanth is a challenge because you want some speed to not be helpless vs Roselia, you want some attack to not be passive in this more-offensive meta game, and you obviously want some bulk. He is also hard to slot on a team; something like Metang or Torkoal are more splashable and fill similar roles.
VenomothOnly really operates as a lead. Is ruined by Lum Berry Glalie. Has a 25% chance to face death vs Torkoal, Flareon leads. Sleep Powder turn one is risky business. I will give Venomoth some credit, they can grant Baton Pass teams and offensive teams a strong start if the Sleep Powder does land. But usually offensive teams just slot lead Glalie and find room for Sleep Powder on a sweeper Bellossom, Hypnosis Chimecho, etc... if they really want sleep. Faces competition as a Baton Pass lead from Minun, Ledian, Shedinja (yes, I think Baton Pass has a lot of niche lead options that can work well).
SudowoodoGood lead with either a Substitute Focus Punch set or Choice Band. Can bust some holes in the early game. A bit hard to fit on a team and justify using over a normal resist with more longevity for the mid-game, or another phys attacker like Hitmonchan as a lead.
TangelaOne of few adequate Dragon Dance Pupitar checks in the tier. Phys. def set can also check Hitmonchan (unless it is bulk up). Tangela has the option to run a sweeper set with Sunny Day. He has more natural speed and power than Bellossom, but his setup opportunities are rarer. I don't think Tangela is bad at all, but they're hard to fit on a team.
YanmaHe's kinda like Reversal Raticate, except he isn't susceptible to being revenge killed by Salac users. However, Yanma lacks the surprise factor that Raticate has. His coverage is imperfect when compared to Raticate, but fighting + flying is historically great and covers most of the bases. To be honest I wasn't sure where to place Yanma.
B- Tier
TropiusSpecialized Hitmonchan check. A grass type that can't check waters (because Ice Beam coverage). But, it is also a grass type that dumpsters Roselia, which is nice. Their flagship Swords Dance, HP Flying, Earthquake, Synthesis set is very fun to use and effective. They just don't fit the mold of the meta game well enough to be a common choice. Specialized.
MawileIn the context of the modern meta game, I would only run SD pass Mawile. That particular set is underrated, and can end some games on the spot. Has problems finding setup opportunity when Wailord, Flareon, Torkoal, Chimecho, etc.. all nail it. Only real safe setup is Roselia, CB normal attackers, and CB Metang locked into Meteor Mash. Still, the SD pass set is so potentially dangerous that it deserves consideration.
WigglytuffBasically a more balance-friendly version of Lickitung. Loses Knock Off, but gains more Spa. Checks sweeper Huntail and Bellossom fairly well with Fire Blast + Thunderbolt coverage and Wish + Protect. Still potential setup food for other threats like DD Pupitar and Calm Mind Chimecho, Bulk Up Hitmonchan; but less so than Lickitung. I wouldn't rely on Wigglytuff entirely to check special threats, they are more of something to use in combination with another less-reliable check (like Spdef Cacturne vs Wailord, they appreciate the wish support from Wigglytuff, also aid vs Chimecho).
LickitungAccess to Knock Off differentiates Lickitung from the other Wish passers. Cool choice on stall. Far too passive to be used on any other team style. Finds itself being taken advantage of by setup threats. I'd pair him with a bulky roar Wailord most often.
SwalotI need to use them a bit more to be certain of placement. Harder to slot on teams than the other poisons. Speed means they are forced to take an Earthquake from Hitmonchan without getting off the first hit (unlike Golbat and Arbok). Checks Sunny Day Bellossom much more reliably than the other poisons. Specially offensive set has potential, as do Rest Talk stall sets. I can see my opinion of Swalot changing to B at some point.
SeadraLead set forces Glalie into a bad position where if he spikes, Seadra gets a free Substitute and 2 hit KO's with Hydro Pump. If Glalie attacks the Substitute repeatedly, Seadra goes into Petaya Berry range and blows Glalie away with Hydro Pump. Glalie cannot force a trade with Seadra lead (unlike Wailord lead), which is why I view his niche as valuable, and not just a lesser Wailord. Beats Flareon, Torkoal, and Metang leads, can chip Wailord very badly with Petaya boosted HP electric. Only B- because his role is small in the scheme of the meta game, but he can be very fearsome in the right game.
PonytaSimilar in function to lead Seadra, but threatens Glalie with an OHKO immediately without Substitute shenanigans. Has a worse matchup spread against the lead metagame at large than Seadra. Loses to Flareon, Torkoal, Wailord. Still beats Metang. Also has access to Hypnosis. Behind a substitute, a Fire Blast, HP Grass, Hypnosis, Petaya Berry Ponyta can cause a ton of damage. Pops off in some games. Only this low because they are specialized.
MightyenaSimultaneously checks Chimecho, Haunter, and Pupitar. Nice combination of traits. Gets taken advantage of by Flareon, Wailord, Roselia, Cacturne, Hitmonchan. Has cool utility moves. Competes with other Chimecho + Haunter checks in the tier for a team slot.
SeviperStrong wall breaker with excellent coverage options. Super fun to use. Problem is that their points of entry are a lot less safe and concrete than say, Crawdaunt. They can come into Hitmonchan fighting STAB, but then chan can nail them with an Earthquake because Seviper is slower than chan. Roselia and Sableye are their safest points of entry. Seviper really craves more speed. Pairing him with a paralysis spreader is an almost necessary measure.
DunsparceUnderutilized. The para flinch set can be a nightmare. He is also the most reliable Haunter Pursuit trapper in the tier with a special set that bluffs the para flinch set. Hard to concretely fit on teams, but his potential is certainly there.
LaironUnique typing gives him a valuable niche. Normal and flying type attacks bounce off of him. Ground coverage destroys him unfortunately, but the same can be said for a lot of normal resists of the tier. Spdef roar set is a good check to Calm Mind Chimecho. You're also a steel type that is neutral to fire moves, and a rock type who is neutral to grass, so you can operate as a Flareon, Torkoal check. Toxic immunity makes a Lefties + Protect set pretty safe to operate.
GravelerOperates similarly to Sudowoodo as a more offense-oriented normal resist with Explosion. Graveler lacks the set versatility of Sudowoodo (Sub Punch/Choice Band) and is obligated to run Choice Band. However, he does have the excellent ground + rock STAB combo that makes Pupitar so dangerous. Graveler's typing prevents him from effective lead usage. In situations where lead Sudowoodo could take a super effective hit, survive, and dish out big damage; Graveler gets OHKO'd (ex: Hidden Power Grass from Flareon). He can be dangerous in the mid-game. Not a bad option.
NoctowlChecks sweeper Bellossom because of his excellent special bulk and Insomnia. Has a role on some spikes teams as a Whirlwind shuffler and Hypnosis spreader.