There was once a comic that didn't even bother to design something: the author took a photo of Florence, made it monochrome and called it a day.
It was something about a saintess being exiled, but I don't remember the title.
There are so many amazing manga fantasy cities that it is hard to choose just one. However, if I had to pick, I would say that Edolas from Fairy Tail is one of the best. Edolas is a parallel world where magic is scarce and people use magical items instead. The city is full of colorful buildings, flying vehicles, and quirky characters. The city also has a dark side, as it is ruled by a tyrannical king who wants to drain all the magic from Earth Land. Edolas is a fascinating and complex city that offers a lot of adventure and mystery.
I believe One Piece is considered a fitting Fantasy Manga... Hmmm... Water Seven, Dressrosa, and the Flower Capital in Wano will always stick out to me! Water Seven Especially, it's beautiful!
Now I do know One Piece bares a weird blend of fantasy and modern aspects, But, I still consider it fantasy