Superhero comics: There are tons of requests for comics by prisoners. Comics are a great way to get casual or non-readers to become engaged with literature and stories because of the association with other worlds and pictures. Comics are surprisingly complex and deal with huge themes which are quintessential in finding moral or heroic associations with real life.
Manga comics are great. Preferably ones printed in English, though not necessary.
Graphic Novels are basically comics, but less about serial issues and more about a one-off narrative. They are usually a little less about heroes and more about complex, regular individuals; though that's not completely what sets them apart from comics. Basically, they're comics that adults read and are called something else so that adults can feel they're intelligent when they say the words, "graphic-novel". But they are children, these adults, and children are all fantastic thinkers when they're reading good stories.