I guess I'm just struggling to view it's potential objectively. Half of the teams you find from 1500 elo + seem like they almost accidentally have at least 3mons which make Hydreigon a complete nonfactor in battles. Properly built HO, Bulky Offense, Balance and Stall teams should never really have a problem with it.
I think I've built plenty of "proper" HO teams and Scarf Hydreigon is one of the absolute worst mons to face. It's one of the reasons I almost always had to check the "strong prio vs Hydrei" box specifically when building (your answers here are Infernape and Mamoswine, neither of which OHKO). Dragon resists are few and far between on HO teams, it's usually something like offensive Empoleon or Cobalion (the latter of which is a semi-useful Hydreigon check because it can "take advantage" of -2 Hydrei). But those are not terribly sturdy responses, and you end up having to plan your game many turns in advanced to avoid getting swept by Scarf Hydrei.
BO teams should have the easiest times vs it with things like MBlastoise and co, but this is where stuff like the LO version become wildly more threatening. BO team staples like Suicune, Entei, Krook, etc. are all not terribly great in front of Hydrei.
Balance is similar, but 99% of balance teams put on Florges or Sylv and call it a day as far as specific Hydreigon checks go. Coincidentally, LO Iron Tail is one of it's best sets.
Stall is probably the worst though. Taunt Roost gives issues to almost every single stall team. Take a look at Christo vs Manipulative recently in UUPL. Christo's team has a Blissey and the Hydreigon doesn't have Superpower and it STILL is a major factor throughout the match.
For those of us that are not elite tournament players, could you please substantiate this? I'm looking at the list and having a hard time believing it. I understand something like Suicune but can't see how you beat Salamence (which outspeeds you if it wants to), Cobalion (which doesn't die from your attack and can OHKO) etc. among others.
Maybe this belongs in the "Other Questions" thread but it would help a lot with education (or even a "Hydreigon FAQ" ... I had been using Hydreigon for months and, while it's good, I have no idea why it's S rank either!)
MAero - doesn't take scarf dracos without having roost (less common right now), doesn't OHKO ever and dies to LO draco like 50% of the time from full
Salamence - scarf set is generally not the best and is rare, so Hydrei's scarf set wins out in 99% of cases
Suicune - Taunt Roost makes Suicune a nonentity
Entei - Hydrei resists STAB and OHKOs after rocks (and like 50% of the time w/o them)
Mega Swampert - Rain Dance / Ice Punch is a bit tricker for Hydrei to be fair, but the bulky ones lose to taunt LO immediately
Alakazam - dies to scarf
Cobalion - dies to scarf
Mamoswine - ada LO shard doesn't OHKO, dies to draco 100% of the time
Gyarados - trickier for the Hydrei player but LO wins because ice fang is a silly option most of the time, you taunt the sub or DD and just pulse
Florges - LO iron tail etc.
Now obviously it cannot be all of these things at once, but that's not what you're supposed to do. When building your team see what the rest of your members struggle with an literally customize this to beat the common sets of whatever it is you don't like facing. There are so many unexplored options its outrageous, even Choice Band is good
Frankly I think it's the best mon in the tier and it's almost comical someone would nominate B-