I'm surprised no one suggested Frieren : Beyond Journey's End yet. I love it so much !
Oh, Black Butler is indeed a nice anime. Though I would recommend watching only Season 1 to get the most pleasant experience. Having new episodes beyond the Season 1 finale ruined it quite a bit for me.
Houseki no Kuni (Land of the Lustrous) is a gorgeous anime. Unfortunately, it stopped just before the tone became drastically different and would have been dope to watch. Though the manga is complete if you don't mind having to read the rest and get a stylistic whiplash at first. (Don't worry, you get used to it if you push through) I did and that was quite the experience too.. I wasn't ready for the existential dread, though. Otherwise, I recommend it !