1. Chester P. Runk and Allegra Garcia
You're probably not surprised to see Chester and Allegra on this list. The truth is that they really shouldn't be. If the show had handled their relationship right, they wouldn't be.
These two had been making heart eyes at each other since season 7 and the show actively chose not to put them together until season 9. Heck, we find out that in an alternate timeline, they still didn't get together until a near-destruction of said timeline forced them to kiss. In the main timeline, though, we had to sit and watch them awkwardly go through all the recycled relationship tropes, deny their feelings, avoid conversations, and so much more. By the time they finally got together, nobody cared.
When they finally did get together, they just went through the most basic of couple tropes, taking valuable screentime away from the show's main characters (and doing a disservice to their own personal arcs).
It's a shame because I really liked Chester and Allegra as characters when they first arrived. I also found myself rooting for their relationship before it even became a thing. But then it didn't become a thing. And then it did become a thing. And all the potential they had as characters was squandered by a romance plot that was somehow both underdeveloped and overexposed.