I would like to nominate...
B+ to
A-/A
Mega Houndoom benefits from not only revenge killing key threats such as Scizor, Cobalion, Serperior and Latias, but also being able to break bulkier Pokemon such as Gliscor, Amoonguss and even Blissey. Truly this shows the viability of this Pokemon. First, let's talk about the faults of Houndoom, so we can get them out of the way first. Mega Houndoom first takes up a Mega slot, while this isn't too bad, strong Megas like Aero, Altaria and Manetric give this Pokemon competition for that slot. Houndoom is weak to every form of hazard in the game, especially Stealth Rocks and with no reliable recovery, it's prone to be worn down. Houndoom's typing defensively isn't great either, weakness to common priority cripples it. These weaknesses lean myself more towards the A- ranking, but I can still see enough potential for it to be A rank material. Here's why:
Firstly, it's a fantastic revenge killer due to it's typing being anti-meta. Fire-type wall-breakers and sweepers are rare, it's only
real competition being Z-Hurricane Moltres and Salazzle. Access to Nasty Plot allows Houndoom to boosts it's strong 379 Special Attack (with speed benefit nature) to greater heights to deal with even the bulkiest of walls. Now at first Salazzle's typing may seem it has a better match-up against Pokemon such as Primarina, Sylveon and Perish Trap/Choice Locked Azumarill, but check this:
252 SpA Houndoom-Mega Sludge Bomb vs. 0 HP / 0 SpD Primarina: 182-216 (60.4 - 71.7%) -- guaranteed 2HKO
252 SpA Houndoom-Mega Sludge Bomb vs. 252 HP / 4 SpD Azumarill: 248-294 (61.3 - 72.7%) -- guaranteed 2HKO after Leftovers recovery
+2 252 SpA Houndoom-Mega Sludge Bomb vs. 252 HP / 0 SpD Sylveon: 330-390 (83.7 - 98.9%) -- 75% chance to OHKO after Stealth Rock and Leftovers recovery
These Pokemon whose typing wall Houndoom's offensive typing, can be dealt with Houndoom's lovely Poison-coverage of Sludge Bomb. Pokemon such as Sylveon, cannot wall Houndoom once it setups on a switch as Sylveon can only do 56% max if defensive with Hyper Voice. Houndoom's decent bulk also sets it apart from Salazzle:
0 SpA Alomomola Scald vs. 0 HP / 0 SpD Houndoom-Mega: 96-114 (32.9 - 39.1%) -- 98.9% chance to 3HKO
+2 252 SpA Houndoom-Mega Dark Pulse vs. 40 HP / 216 SpD Alomomola: 361-426 (75 - 88.5%) -- guaranteed 2HKO after Leftovers recovery
0 SpA Alomomola Scald vs. 8 HP / 0 SpD Salazzle: 128-152 (45.8 - 54.4%) -- 45.3% chance to 2HKO
+2 248 SpA Salazzle Sludge Wave vs. 40 HP / 216 SpD Alomomola: 363-427 (75.4 - 88.7%) -- guaranteed 2HKO after Leftovers recovery
+2 252+ Atk Life Orb Technician Scizor Bullet Punch vs. 8 HP / 0 Def Salazzle: 211-250 (75.6 - 89.6%) -- guaranteed 2HKO
+2 252+ Atk Life Orb Technician Scizor Bullet Punch vs. 0 HP / 0 Def Houndoom-Mega: 152-179 (52.2 - 61.5%) -- guaranteed 2HKO
Those are some notable examples, Salazzle cannot revenge kill SD Scizor after coming in on Stealth Rock whilst Houndoom can. Houndoom can also setup on Alomomola whilst Salazzle must use it's Z-Crystal, otherwise it loses the match-up.
Houndoom is capable of OHKOing these notable Pokemon without boosts or hazards:
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,
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(50% chance without a Spike, 100% with a Spike)
These notable Pokemon are 2HKO'd (or +2 Boost) or OHKO'd/2HKO'd with Rocks/Spike Damage
- (OHKO'd after Rocks with Fire Blast)
- (2HKO'd, 56.3% to OHKO with Rocks, Manetric T-Bolt does 67.3% max. Probably the most unfavourable offensive matchup outside of Mega Aero)
- (93.8% to OHKO after Rocks with Fire Blast which can't miss because of No Guard, Hurricane only has a 56.3% to OHKO with Rocks.)
- (Strong chance to OHKO after Rocks, Latias can't revenge kill if it survives unless Life Orb or uses Z-Crystal. Life Orb has 50% chance to kill.)
- (Bulkier variants still is 2HKO'd by Sludge Bomb, offensive variants can survive one Sludge Bomb with Rocks but can't revenge kill unless with Stealth Rock chip damage or crit)
- (98.4% to 2HKO with Rocks SpDef Glis, can't be revenge killed without Stealth Rock chip damage or crit. Offensive variants can't kill 100% of time without Rocks and Houndoom now has a 99.6% 2HKO after Rocks with Dark Pulse the offensive variants.)
- (2HKO'd by Fire Blast, 86.7% to be 2HKO'd by Dark Pulse after Spike chip)
- (2HKO'd by Sludge Bomb)
- (2HKO'd by Sludge Bomb, only should pick off weakened Choice Lock Azu 68.6% min, safe on Perish Trap)
- (Dies to Fire Blast 100% at 79.7%)
- (2HKO'd by Fire Blast)
- (Fat variants still 2HKO'd after Rocks by Fire Blast)
- (2HKO'd 100% after Spike with Dark Pulse and 98.8% with Rocks)
- (2HKO'd by Dark Pulse)
Finally, Pokemon that are either setup fodder or beaten after some chip:
- Hits are too weak despite Water-Typing, setup fodder
- +2 Fire Blast is a 100% chance to kill at 60.6%, due to Houndoom's Dark-typing, it can throw off a Fire Blast if Muk switches in, then switch out and doesn't worry about Pursuit.
- Even the greatest special wall in UU is setup fodder, Seismic Toss is a 3HKO on Houndoom and +4 Fire Blast 2HKOs Blissey. Assuming they come in on your Nasty Plot, just plot again lol.
Obviously +2 can net a lot of OHKOs/2HKOs with or without rocks/spikes, so I won't go over that list. As you can see, it deals with many of the top defensive and offensive Pokemon of the tier.
Due to it's fantastic speed tier, strong special attack, amazing offensive typing and access to Nasty Plot, M-Houndoom excels at both revenge killing, wall-breaking and sweeping mid-to-late game. It's coverage and ability to boost makes it tough to switch-in on without a sack. Also having access to Taunt and Destiny Bond can provide arch-types such as stall and even offensive teams more problems. M-Houndoom can also run Sunny Day or paired with a Pokemon like Klefki with Sunny Day to boost it's power even more and net OHKOs/2HKOs prior it couldn't achieve. With hazard removal which most teams should have anyways and paired with a Pokemon like Amoonguss (which is super splashable right now) who deals with priority users such as Breloom and Azumarill, M-Houndoom truly shines. Certainly it is on par with at least A- Rank Pokemon, maybe even A Rank. (This was a lengthy post, apologies it's my favourite Mega lol)