I remember Jack & Jill solely for the TV preview in Teen People(?). I knew nothing about the plot, except it was about two very good-looking people who looked great next to a Hard Candy ad.  

For me, it's Ed. It's more infuriating that you cannot even find it anymore — it's been expunged from history: 

https://www.vulture.com/2016/11/strange-disappearance-of-ed.html

Also, more recently, GLOW. SO sad we don't get to see how it ends. 

Definitely Shameless and Genera+ion.. My heart is still broken.

The simple, and correct, answer here is always Freaks and Geeks, the criminally short-lived dramedy which was the launching pad for stars like Seth Rogen, Linda Cardellini and Jason Segel. Sure, it's a perfect, single season but it's deeply unfair that we never got to know what became of the beloved characters. 

However, my bitterness is held deeply for another little dramedy called Jack & Jill, which aired on The CW from 1999-2001 and, presumably, was watched by only me. This show *also* had some up-and-coming talent at the time (including Amanda Peet, Sarah Paulson and Justin Kirk) and ended on a freakin' cliffhanger!! 

You see, Jack was a girl and Jill was a boy and they were attractive people who fell in love and were planning on getting married but then Jill was getting cold feet and Jack was pregnant and she hadn't yet told him and that's it that's how it ended and I never got closure and I never will!!!! 

So which show's cancellation makes you feel the same sense of unresolved feelings?

Which show's cancellation will forever make you bitter?