Gintama

The story focuses on an eccentric samurai, Gintoki Sakata, his (partial) apprentice Shinpachi Shimura, and a teenage alien girl who's from one of the strongest races in existence: Kagura. All three are "free-lancers" search for work in order to pay the monthly rent, which usually goes unpaid anyway. The story takes place in the Edo Period, but instead of the Europeans invading in the late 1860s they are invaded by aliens, which they call Amanto. It's funny, and worth reading just for the laughs. This manga has a plot with no bounds, it can go anywhere with it, and it does, comedy and seriousness from time to time to make this a MUST read.