Tsubasa Reservoir Chronicle

Spoiler: Plot Summary
Wikipedia said:
Sakura is the princess of Clow Country, which is ruled by her older brother, King Tōya. Her childhood friend Syaoran is a young archaeologist. Sakura is revealed to have strange powers when she has a vision of a mysterious symbol and places she has never seen. Meanwhile, Syaoran discovers the same symbol at the ruins he is excavating. He sees Sakura standing on the symbol on the ground. Ghostly wings appear on her back and a mysterious force begins to pull her into the walls of the ruins. Syaoran rescues her in time, but her wings are scattered across dimensions. The High Priest of Clow Country, Yukito, immediately realizes that Sakura's "wings" were the manifestation of her soul and memories; without them, she will die. In order to save Sakura, Syaoran must journey to retrieve her wings' feathers, the fragments of her memories.

Yukito sends Syaoran and the unconscious Sakura to the Dimensional Witch, Yūko, who is one of the main characters in xxxHolic. There he meets Kurogane, a rough-mannered ninja banished from his world by Princess Tomoyo, and Fay D. Flourite, a magician who fled his world to avoid King Ashura. Each of them must pay with what he values most in order to gain the power to cross dimensions. For Kurogane, it is his sword Ginryū, and for Fay, it is the symbols on his back which regulates his control of his magic. (Fay later reveals that he still possesses magic, but has sworn not to use it without the tattoo on his back.) Syaoran, on the other hand, must pay with his relationship with Sakura: even if he is able to retrieve all of her memories, she will never remember anything about him or their relationship. (This sacrifice also pays Sakura's "toll" to Yūko, because what Sakura values most are her memories of and with Syaoran.) Only when the three agree to her terms does Yūko present them with the power to cross dimensions; a white creature named Mokona Modoki.

While traversing through worlds to find Sakura's feathers, the four travellers and Mokona are forced to overcome many dangers and opponents, some of whom are figures from Syaoran's past who wish to collect Sakura's feathers for their own reasons as they are seen to be objects that hold great power.

The manga adopts greatly dark and complex tones later on, including quite drastic changes to most of the characters and growing problems in the main group.

Note that because of the crossover characters with xxxHolic, some of the plot points of the two manga occasionally intertwine with each other.

I've been reading this manga for about 2 years now. it's really good, the story and plot is just amazing and the art is speechless. This series is also known as Clamps "Biggest" Crossover of all time. So if you've read/seen some of the other works Clamp has worked on, then you can probably expect to see some characters/worlds from it in here.

This series is pretty damn popular but at the same time, many people haven't gotten a chance to read it yet, especially people here. It currently has no plans to end yet, the manga is supposed to go on for a great long time. While the anime has finished with 3 OVAs coming out soon to help catch up with the manga.

If you've seen the anime and not read the manga, then hell, you wasted your time the anime doesn't hold a candle to its manga counterpart.

So go grab your hands on it now, you probably won't be dissapointed.