Okey, so I'm not really sure what the public opinion on Tentacruel is, but after doing some extensive calcs against the top threats in OU, it is very hard to justify using Tentacruel over something else, outside of rain.
Tentacruel finds itself hardpressed to counter or even check the majority of S-ranked 'mons. The best matchups are against non-Swords Dance Aegislash and to a lesser extent Char-Y. In order to even have a shot at countering those Pokemon, Tentacruel needs to have a specially defensive set. It can take Aegislash's Shadow Balls comfortably while annoying with Scald, but if it is matched up against a Swords Dance Aegislash, it is bound to lose even with a physically defensive set:
Against Char-Y, the best Tentacruel can do is to stall out the sun, as even with a specially defensive set it struggles to overpower Char-Y. Though Char-Y can usually 4HKO Tentacruel, so stalling is easier said than done.
Outside of these two, Tentacruel needs to have a physically defensive set to counter or check the things its unique typing enables it to check. The only notable special attacker in the A-ranks Tentacruel has a good chance of defeating is a Thunderbolt-less Gengar:
Because of M-Venusaur, Greninja and Manaphy have started carrying Extrasensory, so Tentacruel can't be trusted in handling these threats. Although OP says that Tentacruel has a favorable matchup against "Fairies in general", it is simply not true in most cases. Even though Tentacruel does well against a defensive Clefable, it becomes a setup fodder if Clefable happens to carry Calm Mind:
One could think that Keldeo is a common special attacker Tentacruel could check, but even that's not true for two reasons: as shown in the calcs, Tentacruel is just so damn weak without investment in special attack that it can't outpower anything, and because Secret Sword targets Tentacruel's weaker Defense-stat, it does a number. Against Keldeo, you're better off using either a fully physically defensive set, or some obscure mixed set, but either way you play, your Sludge Bomb won't outdamage Keldeo's moves. If you want to use Tentacruel as a check to Keldeo, you are actually better off using Giga Drain, as it tips the scale a bit in your favor. All in all, Tentacruel fares quite poorly against Keldeo. Even if you could check Specs-Keldeo, you risk being a setup fodder for the CM variants.
Now for the physically defensive set. There are some physical attackers that Tentacruel can check fairly reliably, and those are mainly Azumarill and Talonflame. Azumarill's STABs are bad against Tentacruel, and with Sludge Bomb being supereffective, Tentacruel can usually win against Azumarill. The only Talonflame variant Tentacruel outright loses to is the Swords Dance set, which will OHKO you after a Swords dance regardless.
Outside of Azumarill and Talonflame, you are hard-pressed to find physical threats that Tentacruel can reliably check. M-Mawile usually has a Dark-type coverage move, which Tentacruel really can't take. Same goes for M-Scizor, which will win if it has Knock Off. Tentacruel can only check Scizor if it only uses STABs, or if it is a CB-variant. Usually M-Mawile and M-Scizor carry Swords Dance, and that is something Tentacruel really can't stomach. Tentacruel usually resists Conkeldurr's coverage moves, but you can't really do anything back, and in a worst case scenario, u give Conkeldurr a burn or poison.
As a spinner, Tentacruel usually fares quite well against common defensive hazard setters, mainly Heatran and Skarmory. Heatran doesn't like to eat Scalds, and Skarmory can get annoyed with Scald's burn chance. If using Toxic Spikes, u can usually go against Mandibuzz with relative ease by using Toxic.
As we can conclude, Tentacruel is hardpressed to check the threats that define OU metagame. There are only a handful of Pokemon it can check as a pivot, and even those can't be checked with the same set. U could have a physically defensive set and a specially defensive one, but both of them check such a small number of Pokemon that it isn't really worth it. If u try and split the defenses you start losing your ability to check anything at all. There's also the fact that uninvested Scalds and Sludge bombs are terribly weak, and often you are relying on their secondary effects to check stuff reliably. As Tentacruel can't operate well as a bulky water-type because of its inability to check the things it would want to in one set, and because Rapid spin is nowhere near as mandatory as it once was, I'm seriously questioning whether Tentacruel deserves an OU analysis at all.
Tentacruel finds itself hardpressed to counter or even check the majority of S-ranked 'mons. The best matchups are against non-Swords Dance Aegislash and to a lesser extent Char-Y. In order to even have a shot at countering those Pokemon, Tentacruel needs to have a specially defensive set. It can take Aegislash's Shadow Balls comfortably while annoying with Scald, but if it is matched up against a Swords Dance Aegislash, it is bound to lose even with a physically defensive set:
+2 252+ Atk Life Orb Aegislash-Blade Shadow Claw vs. 252 HP / 224+ Def Tentacruel: 352-415 (96.7 - 114%) -- 81.3% chance to OHKO
+2 252+ Atk Life Orb Aegislash-Blade Head Smash vs. 252 HP / 224+ Def Tentacruel: 500-590 (137.3 - 162%) -- guaranteed OHKO
+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 224+ Def Tentacruel: 200-238 (54.9 - 65.3%) -- guaranteed 2HKO after Black Sludge recovery
0 SpA Tentacruel Scald vs. 0 HP / 4 SpD Aegislash-Blade: 124-147 (47.5 - 56.3%) -- 84% chance to 2HKO
+2 252+ Atk Life Orb Aegislash-Blade Head Smash vs. 252 HP / 224+ Def Tentacruel: 500-590 (137.3 - 162%) -- guaranteed OHKO
+2 252+ Atk Life Orb Aegislash-Blade Shadow Sneak vs. 252 HP / 224+ Def Tentacruel: 200-238 (54.9 - 65.3%) -- guaranteed 2HKO after Black Sludge recovery
0 SpA Tentacruel Scald vs. 0 HP / 4 SpD Aegislash-Blade: 124-147 (47.5 - 56.3%) -- 84% chance to 2HKO
Against Char-Y, the best Tentacruel can do is to stall out the sun, as even with a specially defensive set it struggles to overpower Char-Y. Though Char-Y can usually 4HKO Tentacruel, so stalling is easier said than done.
252 SpA Mega Charizard Y Fire Blast vs. 252 HP / 224+ SpD Tentacruel in Sun: 102-120 (28 - 32.9%) -- 85.9% chance to 4HKO after Black Sludge recovery
252 SpA Mega Charizard Y Solar Beam vs. 252 HP / 224+ SpD Tentacruel: 99-117 (27.1 - 32.1%) -- 54.3% chance to 4HKO after Black Sludge recovery
252 SpA Mega Charizard Y Dragon Pulse vs. 252 HP / 224+ SpD Tentacruel: 70-83 (19.2 - 22.8%) -- possible 6HKO after Black Sludge recovery
0 SpA Tentacruel Sludge Bomb vs. 0 HP / 0 SpD Mega Charizard Y: 72-85 (24.2 - 28.6%) -- 98.3% chance to 4HKO
0 SpA Tentacruel Scald vs. 0 HP / 0 SpD Mega Charizard Y: 128-152 (43 - 51.1%) -- 3.1% chance to 2HKO (outside of sun)
252 SpA Mega Charizard Y Solar Beam vs. 252 HP / 224+ SpD Tentacruel: 99-117 (27.1 - 32.1%) -- 54.3% chance to 4HKO after Black Sludge recovery
252 SpA Mega Charizard Y Dragon Pulse vs. 252 HP / 224+ SpD Tentacruel: 70-83 (19.2 - 22.8%) -- possible 6HKO after Black Sludge recovery
0 SpA Tentacruel Sludge Bomb vs. 0 HP / 0 SpD Mega Charizard Y: 72-85 (24.2 - 28.6%) -- 98.3% chance to 4HKO
0 SpA Tentacruel Scald vs. 0 HP / 0 SpD Mega Charizard Y: 128-152 (43 - 51.1%) -- 3.1% chance to 2HKO (outside of sun)
Outside of these two, Tentacruel needs to have a physically defensive set to counter or check the things its unique typing enables it to check. The only notable special attacker in the A-ranks Tentacruel has a good chance of defeating is a Thunderbolt-less Gengar:
252 SpA Life Orb Gengar Shadow Ball vs. 252 HP / 224+ SpD Tentacruel: 110-133 (30.2 - 36.5%) -- guaranteed 4HKO after Black Sludge recovery
0 SpA Tentacruel Scald vs. 4 HP / 0 SpD Gengar: 91-108 (34.7 - 41.2%) -- guaranteed 3HKO
0 SpA Tentacruel Scald vs. 4 HP / 0 SpD Gengar: 91-108 (34.7 - 41.2%) -- guaranteed 3HKO
Because of M-Venusaur, Greninja and Manaphy have started carrying Extrasensory, so Tentacruel can't be trusted in handling these threats. Although OP says that Tentacruel has a favorable matchup against "Fairies in general", it is simply not true in most cases. Even though Tentacruel does well against a defensive Clefable, it becomes a setup fodder if Clefable happens to carry Calm Mind:
0 SpA Tentacruel Sludge Bomb vs. 252 HP / 0 SpD Clefable: 176-210 (44.6 - 53.2%) -- 28.1% chance to 2HKO
0 SpA Tentacruel Sludge Bomb vs. +1 252 HP / 0 SpD Clefable: 116-140 (29.4 - 35.5%) -- 27.8% chance to 3HKO
+6 0 SpA Life Orb Clefable Moonblast vs. 252 HP / 224+ SpD Tentacruel: 164-194 (45 - 53.2%) -- 0.4% chance to 2HKO after Black Sludge recovery
0 SpA Tentacruel Sludge Bomb vs. +1 252 HP / 0 SpD Clefable: 116-140 (29.4 - 35.5%) -- 27.8% chance to 3HKO
+6 0 SpA Life Orb Clefable Moonblast vs. 252 HP / 224+ SpD Tentacruel: 164-194 (45 - 53.2%) -- 0.4% chance to 2HKO after Black Sludge recovery
One could think that Keldeo is a common special attacker Tentacruel could check, but even that's not true for two reasons: as shown in the calcs, Tentacruel is just so damn weak without investment in special attack that it can't outpower anything, and because Secret Sword targets Tentacruel's weaker Defense-stat, it does a number. Against Keldeo, you're better off using either a fully physically defensive set, or some obscure mixed set, but either way you play, your Sludge Bomb won't outdamage Keldeo's moves. If you want to use Tentacruel as a check to Keldeo, you are actually better off using Giga Drain, as it tips the scale a bit in your favor. All in all, Tentacruel fares quite poorly against Keldeo. Even if you could check Specs-Keldeo, you risk being a setup fodder for the CM variants.
0 SpA Tentacruel Sludge Bomb vs. 0 HP / 4 SpD Keldeo: 88-105 (27.2 - 32.5%) -- guaranteed 4HKO
0 SpA Tentacruel Giga Drain vs. 0 HP / 4 SpD Keldeo: 98-116 (30.3 - 35.9%) -- 34.6% chance to 3HKO
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 0 Def Tentacruel: 147-174 (40.3 - 47.8%) -- guaranteed 3HKO after Black Sludge recovery
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 224+ Def Tentacruel: 100-118 (27.4 - 32.4%) -- 70.1% chance to 4HKO after Black Sludge recovery
252 SpA Choice Specs Keldeo Hydro Pump vs. 252 HP / 0 SpD Tentacruel: 114-135 (31.3 - 37%) -- guaranteed 4HKO after Black Sludge recovery
0 SpA Tentacruel Giga Drain vs. 0 HP / 4 SpD Keldeo: 98-116 (30.3 - 35.9%) -- 34.6% chance to 3HKO
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 0 Def Tentacruel: 147-174 (40.3 - 47.8%) -- guaranteed 3HKO after Black Sludge recovery
252 SpA Choice Specs Keldeo Secret Sword vs. 252 HP / 224+ Def Tentacruel: 100-118 (27.4 - 32.4%) -- 70.1% chance to 4HKO after Black Sludge recovery
252 SpA Choice Specs Keldeo Hydro Pump vs. 252 HP / 0 SpD Tentacruel: 114-135 (31.3 - 37%) -- guaranteed 4HKO after Black Sludge recovery
Now for the physically defensive set. There are some physical attackers that Tentacruel can check fairly reliably, and those are mainly Azumarill and Talonflame. Azumarill's STABs are bad against Tentacruel, and with Sludge Bomb being supereffective, Tentacruel can usually win against Azumarill. The only Talonflame variant Tentacruel outright loses to is the Swords Dance set, which will OHKO you after a Swords dance regardless.
252+ Atk Choice Band Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 224+ Def Tentacruel: 187-220 (51.3 - 60.4%) -- 91.8% chance to 2HKO after Black Sludge recovery
0 SpA Tentacruel Sludge Bomb vs. 164 HP / 0 SpD Azumarill: 194-230 (50.7 - 60.2%) -- guaranteed 2HKO
+6 252 Atk Huge Power Azumarill Play Rough vs. 252 HP / 224+ Def Tentacruel: 315-371 (86.5 - 101.9%) -- 12.5% chance to OHKO
0 SpA Tentacruel Sludge Bomb vs. 4 HP / 0 SpD Azumarill: 194-230 (56.7 - 67.2%) -- guaranteed 2HKO
0 SpA Tentacruel Sludge Bomb vs. 16 HP / 240 SpD Assault Vest Azumarill: 102-120 (29.5 - 34.7%) -- 4.2% chance to 3HKO
252+ Atk Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 224+ Def Tentacruel: 124-147 (34 - 40.3%) -- 46.5% chance to 3HKO after Black Sludge recovery
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 224+ Def Tentacruel: 228-268 (62.6 - 73.6%) -- guaranteed 2HKO after Black Sludge recovery
0 SpA Tentacruel Scald vs. 0 HP / 0 SpD Talonflame: 194-230 (65.3 - 77.4%) -- guaranteed 2HKO
0 SpA Tentacruel Scald vs. 248 HP / 252+ SpD Talonflame: 132-156 (36.7 - 43.4%) -- 97.9% chance to 3HKO after Leftovers recovery
+3 0 Atk Talonflame Brave Bird vs. 252 HP / 224+ Def Tentacruel: 262-309 (71.9 - 84.8%) -- guaranteed 2HKO after Black Sludge recovery
0 SpA Tentacruel Sludge Bomb vs. 164 HP / 0 SpD Azumarill: 194-230 (50.7 - 60.2%) -- guaranteed 2HKO
+6 252 Atk Huge Power Azumarill Play Rough vs. 252 HP / 224+ Def Tentacruel: 315-371 (86.5 - 101.9%) -- 12.5% chance to OHKO
0 SpA Tentacruel Sludge Bomb vs. 4 HP / 0 SpD Azumarill: 194-230 (56.7 - 67.2%) -- guaranteed 2HKO
0 SpA Tentacruel Sludge Bomb vs. 16 HP / 240 SpD Assault Vest Azumarill: 102-120 (29.5 - 34.7%) -- 4.2% chance to 3HKO
252+ Atk Huge Power Azumarill Knock Off (97.5 BP) vs. 252 HP / 224+ Def Tentacruel: 124-147 (34 - 40.3%) -- 46.5% chance to 3HKO after Black Sludge recovery
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 224+ Def Tentacruel: 228-268 (62.6 - 73.6%) -- guaranteed 2HKO after Black Sludge recovery
0 SpA Tentacruel Scald vs. 0 HP / 0 SpD Talonflame: 194-230 (65.3 - 77.4%) -- guaranteed 2HKO
0 SpA Tentacruel Scald vs. 248 HP / 252+ SpD Talonflame: 132-156 (36.7 - 43.4%) -- 97.9% chance to 3HKO after Leftovers recovery
+3 0 Atk Talonflame Brave Bird vs. 252 HP / 224+ Def Tentacruel: 262-309 (71.9 - 84.8%) -- guaranteed 2HKO after Black Sludge recovery
Outside of Azumarill and Talonflame, you are hard-pressed to find physical threats that Tentacruel can reliably check. M-Mawile usually has a Dark-type coverage move, which Tentacruel really can't take. Same goes for M-Scizor, which will win if it has Knock Off. Tentacruel can only check Scizor if it only uses STABs, or if it is a CB-variant. Usually M-Mawile and M-Scizor carry Swords Dance, and that is something Tentacruel really can't stomach. Tentacruel usually resists Conkeldurr's coverage moves, but you can't really do anything back, and in a worst case scenario, u give Conkeldurr a burn or poison.
As a spinner, Tentacruel usually fares quite well against common defensive hazard setters, mainly Heatran and Skarmory. Heatran doesn't like to eat Scalds, and Skarmory can get annoyed with Scald's burn chance. If using Toxic Spikes, u can usually go against Mandibuzz with relative ease by using Toxic.
As we can conclude, Tentacruel is hardpressed to check the threats that define OU metagame. There are only a handful of Pokemon it can check as a pivot, and even those can't be checked with the same set. U could have a physically defensive set and a specially defensive one, but both of them check such a small number of Pokemon that it isn't really worth it. If u try and split the defenses you start losing your ability to check anything at all. There's also the fact that uninvested Scalds and Sludge bombs are terribly weak, and often you are relying on their secondary effects to check stuff reliably. As Tentacruel can't operate well as a bulky water-type because of its inability to check the things it would want to in one set, and because Rapid spin is nowhere near as mandatory as it once was, I'm seriously questioning whether Tentacruel deserves an OU analysis at all.
