The most disappointing on a personal level must have been Charlotte.
I was a HUGE fan of Angel Beats, CLANNAD, Kanon, and Little Busters; I was GENUINELY excited for Charlotte's release.
First episode came out, I loved it. Halfway through the show, I think I remember liking it? And then the show just went balls to the wall running around like a chicken that had its head cut off, aimlessly wandering around. And then the show just sped ran its ending without any organic buildup and robbing the viewer of ANY satisfaction with the ending.
Was never the same since. After that, I dropped Rewrite halfway through the first episode because all the characters just felt so stupid and the episode did a horrible job with introducing me to the world or the characters and what not.
Since then, the most I've done regarding any sort of Jun Maeda or KEY work was host a Kanon rewatch for a Discord server a year or two ago. I liked Kanon, but I DEFINITELY don't place it anywhere NEAR the top of my AniList. And I dropped "The Day I Became A God" a few episodes in because it wasn't interesting to me and it smelled like the same Jun Maeda shit that I experienced with Charlotte (and from what I heard, I dodged a fucking artillery strike of shit). I still have plans on rewatching CLANNAD someday as well, but it's nowhere near my top priority when it comes to rewatches.