Well, Unaware Clefable counters every variant of Dragonite and can potentially work its way around Iron head variants as well.
In fact, unware Clefable is a great Pokemon to use in general as it stops the likes of SubCm Keldeo, CM Mega Sableye and stuff like Mega Lopunny and Mega Gallade as well which make it not only a great Dragon Dance Dragonite check, but also a formidable cleric on balanced teams.
Because of unaware, it ignores dragonite's dragon dances thus forcing them into uselessly boosting in front of Clefable and easily losing their Dragonite to this monstrosity
Despite Iron Head DD Dragonite being a rare sight, say you were to run into for some odd reason, it would still fail to even 2HKO Unware Clefable
252+ Atk Dragonite Iron Head vs. 252 HP / 252+ Def Clefable: 172-204
(43.6 - 51.7%) -- guaranteed 3HKO after Leftovers recovery
Due to this, Unaware Clefable is one of the only Pokemon that can take down a boosted dragonite regardless of how much it has boosted itself, so if there's a Pokemon that can reliably take down Dragonite and check multiple threatening Pokemon as well all in one, I would say Unaware Clefable should be your best bet.
Unaware Quagsire works in a very similar fashion as it can just sit there and spam recover and Toxic till Dragonite eventually confuses itself and dies to Toxic damage. However mixed variants can defeat Quagsire due to unaware also ignoring the stat drops from repetitive Draco Meteors.