Oh, definitely Dragon Ball Z and One Piece. I love how I can just keep coming back to both of those, especially One Piece because there isn't 100+ chapters on epic fight scenes *cough*Bleach(I still love you Kubo-sensei)*cough*. Also I wholly agree on Soul Eater, and also Beauty Pop. Those two, I can read over, and over again. And of course Black Butler pft. and D Gray Man.
My favorite manga would be Mysterious Girlfriend X, Magical Knights Rayearth, Kimikiss- Various Heroines, Ga-Rei-Zero, Psychic Academy, Lets Lagoon, D.N. Angel, Clannad, K-on, Angel Beats, and ofcourse a mainstream one- Bleach