Well

There isn't really a perfect line weight. It's best to actually have varying weights throughout the piece to portray energy and depth. Thicker outlines for parts that are closer to the viewer, and thinner lines for objects further away. The same weight for everything usually makes an image flat and lacking depth. Just play around with stuff until you get the hang of it. Some artists prefer to have a reallllly thin weight, so its more of just a guideline than anything else, but others like thick cartoony ones... whatever works! :P