So I heard a couple of lads say klinklang has no counters since it can run hp grass for quag, it's "only" counter...
First thing I want to make clear is that klinklang is a sweeper, it's not coming in to make significant damage and switch out unless you're in a late game tight situation in which case about any pokemon can do significant damage and klinklang still probably prefers setting up anyway.
Next, I want to make a point clear about wild charge and hp grass which I already made in a past post. Klinklang can run those moves, but really there is absolutely no move worth saccing for hp grass. I think we can all agree, running it is just a stupidly inferior option (unless you're running a weird specs set which is clearly not S rank worthy) and therefore no one should be afraid to switch in a quagsire on klinklang and the "it has no counters" argument is not valid. Next, wild charge is a lot better than hp grass and is viable, but once again, it's not exactly as good as return most of the time. It may help you nab the kill against some water types and arguably opposing klinklang but in all other situations, you just have a weaker coverage move with recoil on the mon that already hates rocky helmet more than any other in the tier. So, while your opponent might have to think twice before switching in his poliwrath on your +1 klinklang, he probably can do so without too much risk and even if he does get his predicts wrong, you just lost probably more than the third of your health killing it and if you didn't, he's probably still alive to get in a vacuum wave.
Now if we put the peices together here's what comes out: Klinklang's best set is the standard GGrind/Return/Sub/Shift Gear one and he's a sweeper that takes at least one turn to set up (no matter how easy it may be to set up, he still needs a turn to do it) therefore he will be looking for OHKO's much more than 2HKO's unless the mon can't break a sub. So, here's a set of potential ways of stopping/revenging klinklang's best set or at least things that are safe to switch in while he shifts his gears.
Quagsire
Gourgeist-Super (+1 252+ Atk Klinklang Gear Grind (2 hits) vs. 248 HP / 200+ Def Gourgeist-Super: 146-174 (39.1 - 46.6%) -- guaranteed 3HKO after Leftovers recovery
+1 0- Atk Gourgeist-Super Foul Play vs. 136 HP / 0 Def Klinklang: 126-149 (42.7 - 50.5%) -- guaranteed 3HKO after Leftovers recovery)
Prinplup
Rotom and Rotom-S
Rhydon (+1 252+ Atk Klinklang Gear Grind (2 hits) vs. 0 HP / 0 Def Eviolite Rhydon: 256-304 (72.9 - 86.6%) -- guaranteed 2HKO)
Garbodor
Claydol
Sandslash
Weezing
Poliwrath
Stunfisk
Lanturn
Combusken
Lipeard (risky switch-in but can encore sub/Shift Gear and sucker punch to revenge kill)
Samurott
And that is just to name the ones with higher usage (I skipped stuff like torkoal, monferno and bulky psychics with hp fire), heck if klingklang hasn't set up twice something as frail as a pyroar after SR can easily stop him...
+1 252+ Atk Klinklang Return vs. 0 HP / 4 Def Pyroar: 198-234 (63.2 - 74.7%) -- guaranteed 2HKO. In fact, it's simple enough to wall klinklang that you can probably surprise him with gimmicks (read rocky helmet) much more than you can surprise your opponent with klinklang. To those who wish to inform me klinklang can also run a Resttalk/GGrind/Shift Gear set, please notice the the great majority of the previously listed mons don't really care.
Oh did I mention his STAB makes contact TWICE and has 85 acc?
In the end, klinklang is not very hard to prepare for, not very useful early-game (or as long as he's not sweeping), not fully reliable, not hard to predict (unless specs is really a thing I guess) and not OHKO'ing that much at all. It's simply not S tier material as long as your opponent isn't stupid enough to try putting it to sleep with his vileplume and grant you that free turn klinklang needs to get that extra set up ("but hey maybe he was running hp grass over sub"
).