im looking for a manga that has an engrossing and well developed fictional world

I know what you mean, but honestly I can't think of anything where the whole world is developed well. These mangas, though, have parts of their world developed well. It's the best I can do. Sorry.

Inuyasha (not necessarily well developed, but I did find this world oddly engrossing)
1/2 Prince
Akagami No Shirayukihime (not super original but some of the political systems are interesting)
Tsubasa Reservoir Chronicles (this one involves many worlds; even so, every thing about this manga is well developed, so it's worth a shot)

I don't know if you're into books that aren't manga, but if so, The Name of the Wind by Patrick Rothfuss has a wonderfully developed world (and I hear so does Widdershins, by Charles deLint)

Id
Berserk especially in the "fantasia" arc.

Aria and its prequel, Aqua. :3

[img]https://i.imgur.com/cWeUK2w.png[/img]

Quote from LawX

You are like the dense main character in a shoujo manga.

Quote from Crenshinibon

And you will murder someone one day, pika. If you're my daughter.

Eden: It's an Endless World!, very realistic and detailed science fiction, somewhat dystopian, version of the future.

Dragon Ball, though it doesn't quite go into as much detail about its world than one would hope. Still interesting though.

Fullmetal Alchemist, very detailed world, deals a lot with world politics and government

SandLand. Only one volume, but the setting is quite interesting.

Berserk, one of the best dark fantasy worlds out there, manga or otherwise.

Claymore. Second only to Berserk.

One Piece. One of the few shounen mangas that's actually epic

Check out my manga podcast:
https://youtube.com/channel/UCr0Us8mMIpX3x0pOud2AUWg

The Twelve Kingdoms has a very well developed and engrossing fictional world. It's not a manga, though; it is a series of books and an anime based on the series.

Kino no Tabi, Ghibli's movies ( Personally I love The Cat Returns, Ponyo, Spirited Away, etc), Daremo Shiranai, Magic Knight Rayearth, avatar: the last air bender.... not forget to mention Fairy Tail.
Second One Piece, FMA, Aria, Tsubasa Reservoir Chronicle

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07-GHOST fits that. It's one of my favorite too. c:

Dorohedoro

So many amazing options to chose from.

One Piece is the first to come to mind... but I'm sure almost everybody has already read that for forever.
Toriko is another cool one from Jump. Though the food-animals do get quite silly at times.
Vinland Saga is a historical manga about The viking Ages, so it's really cool to be able to pull out an actual map and find where the characters are in actual time.
Tegami Bachi. Amberground is so interesting, and beautiful (atleast Yodaka Inward)
Emma I guess I can't go a list without recommending Emma, I see Victorian England in a whole new light now. (not to mention Mori-sensei is the master of "Scenery Porn", see tvtropes.org)

Honorable Mentions
Kisswood
Trace
Magician (KIM Sarae)
Akumetsu
Uzumaki
Maoyuu Maou Yuusha (is only 2 chapters in but looks to be just what you're looking for)

thanks for the recommendations
id, vinland saga, eden, aria, Dorohedoro and berserk seem to fit the criteria perfectly

Aqua / Aria is a slice of life sci-fi. Almost the entire point of the series is learning about the world and the culture, which isn't too different from here, but still has its many nuances. Similarly, you can check out Yokohama Kaidashi Kikou, which does the same thing in a different futuristic setting. I'm going to go ahead and recommend The Music of Marie as well. Which is overall just a really thought provoking story.

If you're looking for something more action-y, I'll second Id. Avoiding any real spoilers, the story centers around a boy who has traveled to another world. Both worlds are fantasy settings, so its not like inuyasha or something, where one of the worlds is "normal". And you get quite the introduction to both worlds as the story progresses.

Saver

The meaning of life...is to give life meaning.