My favourite Manga all-time!
Princess Sakura and Archeologist Syaoran are in love with eachother since childhood, but were never able to confess. When finally deciding to Sakura hears a voice coming from the ruins of her country and suddenly passes out, losing all her memories. Her life was in danger and Syaoran wanted to do everything to get her memories back, even if it meant she would never remember him again. A long adventure throughout the dimensions takes it's place.
Tsubasa: Reservoir Chronicle