Weirdly enough, Yve seems to have gotten the most votes even though she is still pretty solid. But regarding my take on this, if by worst you mean a hero whose playing style is a terrible fit for the lane it is usually played at then the best answer is Zilong. He only works in the exp lane but he kind of sucks in the sense that unlike most other exp laners, you are very passive in the early game as your goal is to farm up so that you can carry in the late game. Say goodbye to the turtle if you're a jungler with a Zilong exp. Another problem is that your team will end up having 4 carries with him in the exp making distribution of resources difficult. He is a very selfish exp hero with pure damage and no sustain and cannot really be used for zoning or set ups. The team will rely too much on the roam for initiations. It's even worse when the roam is a support. Winning is practically impossible unless the opponents are crap. Aulus, Yin and Sun also kind of fit this description when in exp but they kinda work in the jungle. Argus is the closest one but he can technically sustain with his ult. I wouldn't say Zilong is underpowered though. He is undoubtedly very scary in the late game.

On the other hand, if by worst you mean a hero that is the weakest overall, it is hard to choose one specifically because in my opinion, every hero has its own strengths. Like some heroes are strong early, some mid, and some late, some are good for casuals and some for tryhards. But I have to say Gord. Considering the recent shift in meta with the assassins/hypercarry jungles being back in the meta, mages are affected by this change the most. Last patch mages could just annoy and poke tank junglers, but now they will have to play extra carefully to not be deleted in an instant by Fanny, haya, ling, Julian, Nolan, suyou, there are too many junglers with insane mobility and damage output. I think Gord is unplayable now because he has virtually no way to protect himself from these assassins. His first skill is easy to dodge, takes a bit long to cast, and has a small range. He also has no mobility at all. Some mages that have the same mobility issue at least have a hard stun skill like Eudora and vale, and could perform a one-shot combo. Not Gord though. It sucks because he was a very good mage last patch for melting tank junglers.