I really can't say what route is the worst but if I had to field a contender I would say Karin's route in To Heart 2. Any potential sweet moment or anything like that was reduced into nothing thanks to her playing the blackmail card. Repeatedly. And tricking someone to join your club when they help you (even if the story handwaves this as a common occurrence) and blackmailing someone due to an accident into stay in your club is simply wrong.
Other contenders consist of most of the routes in Tick! Tack! are pretty bad mostly because they are based around the concept of "Alright how can we screw up Nerine and how can we screw up her relationship with Rin?"
And there are probably other routes that I think are terrible but I can't think of right now, I will say off the top of my head that I didn't like Nemu's route in Da Capo or Kiri's route in Canvas.The former because it makes me believe the MC and Sakura are idiots and the latter due to how flat and predictable it was. Mare's route is also there but I just think that due to the common reason of it being wasted potential but overall I didn't have too many problems with it.