There is none
The "worst" Dragon type Pokemon is very very subjective to personal bias an opinion. From a stats view, Noibat is the worst with a BST of 245. However, at their stage, the difference in stats is not significant, especially factoring in that they can only be used in LC viably, where they are all set to level 5 - so alot of the time, a slight difference in base stats will cause no actual difference in the real stat, due to the low level. Noivern also trumps most of the other LC Dragons in terms of Speed and has high base power STAB attacks to make up for the lack of offensive power. All their abilities also have their own slight perks.
It's up to personal opinion really - personally, I would say Goomy, since it's abilities are generally inferior to say, Strong Jaw, Mold Breaker, Infiltrator etc. Additionally, it's move-pool is well... acceptable at best. Tbh, it's a pretty barren move-pool - it lacks setup moves, and strong STAB barr Draco Meteor, which lowers your SpA anyway (At least Noivern gets like. Hurricane). This forces it to often take a more defensively based role, however even in this role it gets outclassed by the likes of Vullaby who has much better overall bulk.
However out of fully evolved Dragon Types, it is generally considered by most of the community that either Altaria or Druddigon are the worst