One Piece is my favourite by far. The character development, the story each character has and their respective dreams, their unique traits, the comedy, the epic fights, the emotion it has... it's simply the best. No other manga/anime was able to move me, I cried like crazy on Arlong Park and Enies Lobby (an almost at Drum Island). It's an excellent story, and it's soo funny too! That's why I love it
Others I love are, in order, Fairy Tail, Highschool of the dead, Full Metal Alchemist, Sakura Card Captors, Death Note, Pandora Hearts, Bleach, Kuroshitsuji, Naruto, Soul Eater, Shugo Chara and others I can't recall right now... but these are the best ones

OP is still my clear number 1 though.