I'd say for me it's Erased. The first 3 episodes weer very atmospheric and pretty good overall, but after that it just turned bland. People say that it's a great show with an awful and predictable ending, but for me the meh part started way before the ending.
Fushigi no Kuni no Shounen Alice is a manga that nobody ever talks about, and it has some awfully low rating on Mangaupdates, but I personally found it strangely fun. There isn't really any coherent story in it, neither there is any obvious hidden meaning, and it feels like some kind of a high trip. According to the last chapter, it looks like it's supposed to represent mental illness in some way. Still, somehow, it was really fun to read. I guess, my tastes are just that awful.
Well, here's my personal unpopular opinion: there are quite a lot of really good isekais out there, especially comedies, but they rarely get noticed because, due to the abundance of awfully written escapist fantasies, they rarely ever get noticed by the general public.